The Synchronized History of Ireland
The Book of Invasions (Lebor Gabála Érenn) is part of what modern scholars call the Synchronized History of Ireland. The early Christian monks in Ireland were sponsored by, and generally part of the dominant families of Ireland and they sought to synchronize the various regional stories that were told by the Poets (Fili) into a unified story that would serve two purposes.
One purpose was to turn these often-contemporaneous stories into a linear narrative that could be synchronized with the stories of the Roman, Greek and Hebrew world, about which they had recently become aware. (They used the Hebrew calendar for their dating.) They cut, moved and generally used or ignored whatever they needed to achieve their purpose which was not to record pure history, but rather to join the story of their people to the greater world.
The other purpose was to make their own group the dominant one in Ireland so that they could claim as much power as possible in their time. The monks had an agenda and it was not history. Saint Columba was a prince of the Northern Uí Néill and this was not unusual.
The stories that have come down to us were developed by the two branches of the O’Neill dynasties, the Northern Uí Néill and Southern Uí Néill who ruled most of the north and center of Ireland from around 450 AD. They monopolized the High Kingship of Ireland for 500 years to around 1000 AD. They originally called themselves the Féini as seen in this Eighth Century legal tract.
"There were three principal kinships in Ireland: the Féini, the Ulaidh (Ulster), and the Gáilni, i.e., the Laighin (Leinster)."
Féini was later used to refer to all free peoples, so the Synchronists invented an ancestor called Gaedheal Glas from whom the Uí Néill were said to descend and they became the Gael. Since they wished to rule all of Ireland, their Synchronist monks grafted all free tribes onto their new genealogy and everyone became a Gael to the Uí Néill, even peoples like the Dál Fiatach and Dál Riada who were considered to be Érainn and part of the Ulaidh (Ulster).
For the past 50 or more years, one of the main objects of the study of Irish History is to undo the work of the Synchronists. There well-known scholars were among that group and the first two books have been available since the 1970s
Ireland Before the Normans by
Donncha O'Corrain.
Ireland Before the Vikings by
Gearoid MacNiocaill.
Early Medieval Ireland, 400-1200 by
Daibhi O Croinin