He distinguished four separate waves of Celtic invaders:
In Britain these Priteni were absorbed by later invaders and lost their cultural identity, except in the far north where they were known to the Romans as Picti, or “painted people,” on account of their practice of decorating their bodies with tattoos (a practice which by then had died out among other Celtic nations). In Ireland, too, the Priteni were largely absorbed by later settlers; but a few pockets of them managed to retain a measure of cultural, if not political, independence well into the Christian era. By then they were identified as Cruithne or Cruthin, a Q-Celtic adaptation of the P-Celtic Priteni. Both words are derived from a root meaning “to shape” or “create.” Celtic tribes generally gave themselves names which were the pluralised forms of names they gave to their deities (in this case “the Creator”). Among the Cruthnian tribes that survived into the Christian era the most prominent were the Dál nAraidi in Ulster, and the Loíges and Fothairt in Leinster. The name of the second of these tribes, modernized as Laois, has been revived and given to one of the counties of Leinster (formerly known as Queen's County).
Among the more prominent Ernean tribes were the following:
The Laginian invasion made little impact in Ulster or Munster, where Ernean tribes continued to be the dominant force. But the same cannot be said for Connacht, the westernmost of the four provinces. Sometime in the third century (all these dates, it should be pointed out, are highly conjectural) they crossed the River Shannon and subjugated the Ernean tribes of Connacht. The decisive battle was fought in County Sligo, in a place called Mag Tuired (Moytura). There a Laginian king (possibly known as Cairbre) overthrew the Érainn and drove them out of Connacht. According to Irish records the defeated Érainn sought refuge in many of the islands around Ireland. The fortresses of Dún Aengus and Dún Conor on the Aran Islands, and Dún Balor on Tory Island, are thought to have been built by them.
It was probably as a result of the Laginian conquests that the island of Ireland first came to be divided into four provinces. The Érainn continued to rule in Ulster and Munster, while the Lagin and their allies became the dominant force in Leinster and Connacht. Traditionally these four provinces met at the exact centre of the country, which was marked by the Hill of Uisneach (between Mullingar and Athlone in County Westmeath), a name which may mean “vertex” or “angular place.” The district immediately surrounding this hill was originally called Medion, which means “middle,” and is the origin of the county-name Meath. Julius Caesar informs us that the druids of Gaul regularly assembled at a hallowed spot in the centre of the country to celebrate their rituals (De Bello Gallico 6.13). Irish tradition records that a similar assembly, the Mórdáil Uisnig, periodically took place at the Hill of Uisneach on Beltane, the May-day festival.
The Connachta were named after Conn Cétchathach, or Conn of the Hundred Battles; a mythical ancestor who was later euhemerized and given a place in Irish history. The Connachta were led by a man known to later history as Tuathal Teachtmhar. They landed at the mouth of the Boyne and pushed inland to Tara, the seat of the local Ernean king, which they sacked. They soon carved out for themselves a new province between Ulster and Leinster, running from the mouth of the Liffey to the mouth of the Boyne and inland as far as the Shannon. In time, this fifth province came to be known as “Meath,” presumably for no other reason than that it contained the Hill of Uisnech.
The other group were known as the Eóganachta. Their leader is known to history as Mogh (or Mug) Nuadat. The Eoganachta landed at a place called Inber Scéne, usually identified with Kenmare River in the southwest of the country. Unlike the Connachta, they did not carve out a new province for themselves. Instead, they slowly but gradually rose in power, eventually becoming the dominant force in Munster. The names Eóganachta and Mug Nuadat, which were probably adopted after their arrival, suggest that initially there were friendly relations between them and the Érainn, as both names are derived from the titles of Ernean deities. Early in their history, the Connachta subjugated the Laginian tribes of Leinster and reduced them to a state of vassalage. The Laginian leaders were allowed to retain possession of their territory, but a heavy tribute was imposed upon them. Known as the Bórama (or Bórú), it continued to be exacted at irregular intervals until the eight century of the common era. According to one tradition, it was Tuathal Techtmar himself who first imposed this tribute on the Lagin.
As their name suggests, the Connachta did not stop when they reached the Shannon. At an indeterminable point in history, some of them crossed the Shannon and conquered the western province, bestowing their name on it in the process. By the late fourth century of the common era Ireland was divided politically into five provinces or “overkingdoms”:
As can be seen, most of the country was in the hands of the Gael. Only Ulster remained independent, but this was not to last.