The O’Hearn family appears from the best sources available to originate with the family of Ua hEachthighearna or alternatively Ua hEachthigheirn meaning the descendants of Eachthighearna, horse-lord. The Anglicized form of the name is Echtigern (Welsh ‘Edeyrn’?), similar to Vortigern (Wyrtgeorn, Gwyrtheyrn, Gurtheyrn), king of the Britons of Arthurian legend, and also the 6th Century St. Kentigern (Cyndeyrn) of Scotland, and the 8th Century St. Kentigerna of Ireland and Scotland. Echtigern was a son of Cinneidi, King of Thomond (North Munster), and his wife Babhion ("Fair Maid"), daughter of Arcadh, son of Murchadh (Murrough) O’Flattery, King of Iar Connaught (West Connaught), and brother of the great Irish ard ri (high-king) Brian Boru. Another brother or half brother Mahon had risen to the rank of king of the Munster province before Brian became high-king during the Viking wars about 1000 AD.

The people of their tuatha or petty kingdom styled themselves as Dal gCais or race of Cas, claiming descent from an ancestor who was supposedly related to the southern ruling family elite called the Eoganachta who ruled from their stronghold of Caiseal in Tipperary. In actual history however, they were of a people called the Deisi or vassal people who apparently acted as mercenary soldiers to the more powerful kings of both North and South.

Some originated around Deece near Tara, and migrated south into the Decies of East Cork and Waterford before the Christian era began. Perhaps they served as Irish foot-soldiers called ceithernaigh or kerns. St. Declan, originally from Leinster, became their patron saint. These people came originally from the midlands of Ireland and consequently appear to most closely represent the ethnic group descending from the original hunting, fishing and food-gathering folk who crossed into Ireland from Scandinavia during the Ice Age about 6000 BC, their descendants called Cruithne in Ireland, ethnically similar to the Picts of Alba in Britain, the Lapps in Finland, and possibly also the Guanches of the Canary Islands and the Basques of the Pyrenees Mountains in Spain and France. They may have been absorbed to some extent by the later arrivals who were called Erainn by the Gaels, possibly originally from the Caucasus Mountain region of Iberia or Georgia. These include neolithic farming and pastoral folk, from the Middle East (related to the Aryans) and North Africa (related to the Berbers), arriving about 3000 BC, and the bronze-using precursors of the Celtic peoples arriving from central Europe between 1500 and 800 BC.

Recent antiquarians, including T.F. O’Rahilly, classify the Deisi and the Dal gCais as belonging ethnically to the race of Erainn. At the time of the arrival of the iron-using Celts who settled along the coast beginning before 300 BC, the remnants of the earlier groups remained in the mountainous midlands, much as in the Scottish highlands where traditional culture has been preserved even to modern times.

These successive ethnic migrations are recounted in Irish mythology in An Leabhar Gabhala (The Book of Invasions) naming the settlements of Parthalon, the Nemedians, the Fir Bolg (probably Belgic Erainn), the Tuatha de Danann (who were actually non-material deities), the Fomorians (probably Phoenician or Scandinavian invaders), and finally the Milesians (related to Mil of Spain). This account is somewhat corroborated in the Celtic, Greek and Roman names for Ireland: Eriu, Eire, Iverne, and Hibernia, clearly similar to the name of the Iberian people. However by the dawn of the Christian era, all of the inhabitants of Ireland spoke a common Celtic language and shared in a common Celtic culture. It is interesting to note that the early Irish monks traced the lineage of all of the Irish as Celts from the Scythians, descending from Magog, son of Japheth, son of Noah, and through the line of Seth from our first parents Adam and Eve as appears in Genesis, Chapter 10.

According to one Irish scholar le Poer, the Deisi acted in relation to their Celtic overlords much as the people in the frontier regions of The Roman Empire called laeti served under Roman military commanders. The Dal gCais were in reality an obscure tributary group called An Deis Tuaisciert (the Deisi of the North), a branch of In Deis Becc (Little Deisi) of Munster who occupied a territory in southeast Limerick called An Deis Bheag about the time of St. Patrick’s mission to Ireland in 431 AD. They crossed the Shannon River from Limerick early in the 5th Century AD to acquire new territory and establish a degree of independence from their Eoganacht overlords.

About this time, some others of the Deisi people in the area of Waterford crossed over into Wales and Cornwall in Britain. Among these are perhaps the tyrant Vortigern mentioned above, and Carredig who was lord of what was to become Cardiganshire.

Cinneidi’s father Lorcan mac Lactna was called king of the Dal gCais and claimed descent from the 7th Century chiefs Bloid and Turlough, related historically to two Irish saints, Munchin and Flannan. Bloid’s descendants styled the Ui Bloid were settled in a region in southeast Thomond just north of the Shannon River called Killaloe, or the Church of St. Molua who was an early 7th Century Irish monk of the Ui Fidhgheinte from County Limerick. St. Munchin is the patron of the city of Limerick. St. Flannan, who was consecrated Bishop of Killaloe by Pope John IV in Rome, is patron of the Flannan Islands in the Hebrides off the coast of northern Scotland where he had gone as a missionary.

A tradition is preserved that the Flannan Islands are called the Seven Hunters because of a prophesy that among Turlogh’s descendants the Ui Toirdealbaigh would be seven great kings later to be identified with Brian Boru and several of the later O’Brien rulers. Turlough, who was the father of St. Flannan, also became a monk at Lismore monastery in the south of Ireland.

The Dal Cas territory of Thomond was centered in what is today County Clare, which had been part of the traditional northern half of Ireland called Leth Cuinn (Conn’s half) under the rule of the powerful Connachta. The Deisi Becc people had managed to wrest control of this territory at a time after the Ui Neill dynasty had established power in the midlands or Meath, partly from lands within the southern half called Leth Mogha (Mug’s half), claimed traditionally by the Eoganachta of Munster and the Laigin of Leinster.

The Ui Bloid branch of the Dal Cas rose from obscurity to power in the south of Ireland during the 10th Century AD, at a time when the once powerful Ui Neill alliance, claiming descent from the northern Connachta kings, and the Eoganachta alliance in the south, were relatively weakened due to Viking encroachments throughout Ireland.

The Dal Cas kingdom was strategically located at the mouth of the Shannon River near the newly established Viking stronghold of Limerick. Because of their strategic location and superior fighting skills, they became the most powerful force in Munster in the mid-10th Century, apparently rising with the tacit approval of the Ui Neill rulers. The Ui Neill Ardri Donnchadh selected for a wife Orlaith, a daughter of Cinneidi. In the year 940 AD, she was put to death for sleeping with Donnchad’s son, Aongus, a penalty that may have been imposed because Cinneidi was then perceived as a threat to the Tanaiste or heir apparent Muircheartach (Murtagh). In the year 950 AD, the high-king Conghalach entered and plundered Dal Chais, and caused to be killed two of Cinneidi’s son’s who had enlisted in the Eoganachta army of Ceallachan. These were Donncuan, Tanaiste of Thomond, who was ancestor of the Lonergans, Gunnings, O’Kennedys, O’Regans, Kellihers, among other families, and Echthighern who was the ancestor of the O’Aherne (O’Hearn, etc.) and MacCraith (McGrath, Magrath, etc.) families of the south of Ireland.

Some of the MacGrath descendants later became hereditary bards and historians for the O’Brien kings, as will appear later. They also excelled in the ecclesiastical sphere in both Clare and Limerick. According to An Leabhar Muimhneach (Book of Munster), Echtigern was the father of three sons: Floinn who was the father of Raith from whom the MacCraiths descend; Conghal (Conall) from whom the McConnells descend, and possibly also the MacColes (M’Cole), whom Wolfe says are extremely rare; and Cionath (Kenneth) from whom possibly descend the O’Quirks of Munster. However, another pedigree lists the O’Quirks as descending from Anluan, who was possibly a son of Mahon, son of Cinneidi. The family of Quirks were anciently chiefs of Muscraigh Breogain in Muskerry (Tipperary), which was after the English settlement in the territory of Clanwilliam, a branch of the Norman Burkes. Another family of MacQuirks originates in Ulster.

In A Little Book of Irish Family Names by Ida Grehan, the surname Ahearne is derived from a Gaelic name which became O’Hagerin in English which was later changed to Ahern or Hearn. This was a Dalcassian sept which migrated from east Clare to County Cork. "In the 10th century this was an important sept, whose chieftain was Mathghamain, King of Thomond, an elder brother of the Irish High King Brian Boru. The territory of Thomond at that time covered Co. Clare and parts of Co. Limerick."

This may cause some difficulty since they were all blood siblings and the chief of a clan or sept must have been lineally descended from the ancestor. The difficulty is overcome if one conjectures that Mathghamain (anglicized Mahon) adopted the younger children of Echtigern thereby becoming the chief of the newly formed sept. Moreover, in the Book of Munster, Raith who was the ancestor of the MacRaith sept, is listed as a descendant of both Echtighern and Mahon. The name Raith means Prosperity as listed for the McGrath surname in Ida Grehan’s book.

King Cinneidi was himself killed in 951 AD in battle against the Eoganachta and their Danish supporters. His eldest son Lactna reigned until he was also killed in 954 AD. Mahon, then the oldest son and ancestor of several families including Boland (although Rev. Woulfe says that this sept from a Norse personal name descends from another Mahon, son of Turlough), Spillane, Hanrahan, Sheehan and Toomey, rose to become king of Munster in 954 AD reigning from Cashel of the Kings. He was killed in 976 AD by two supporters of the southern dynasty, Mael Muad (Molloy), and Donovan of the Ui Fidhgheinte, and Ivar the Dane. One other brother Marcan was a monk and abbot of Killaloe, and Bishop of Emly from 990 AD. The scepter thus fell to Brian Boru, who was challenging Ardri Mael Sechnaill (Malachy) II O’Melaghlin of Meath.

King Malachy, while on an excursion south into Leinster in 983 AD, had come into Munster and uprooted the royal tree of Maigh Adhair sacred to the Dal gCais in the inauguration of kings. Brian invaded Ossory in that year and won control of the southern half of Ireland from this this high king Maelsechlainn. Brian rose to the rank of Ardri or high-king in 1002 AD. Brian was killed by the Viking Brodar at the Battle of Clontarf on Good Friday in the year 1014 AD, where a victory was nevertheless decisively won over the Vikings, primarily Danish and Norwegian, and their Irish supporters. This caused the Vikings to give up their plans of conquering Ireland. The descendants of these Vikings who remained in Ireland became thoroughly Irish and were thus absorbed into Irish culture.