The territories of the Southern Ui Néill lay in the counties of Meath, Westmeath, Longford, King's County, and Kildare; they were not continuous, being merely appropriated portions of the kingdom of Tara.
Connacht extended eastward to the Erne and its lakes and to Loch Ramor in Co. Cavan.
Munster comprised its present extent and also the two southern baronies of King's County.
The northern boundary of Leinster ran by the Liffey, its tributary the Rye, south of the barony of Carbury in Co. Kildare, and included part of King's County bordering on Queen's County and Kildare.
There were then seven chief kingdoms in Ireland, each of them containing a number of minor kingdoms. The seven chief kingdoms were (1) the kingdom of Tara, the midlands east of the Shannon; (2) the kingdom of Leinster; (3) the kingdom of Cashel or of Munster; (4) the kingdom of Cruachain or of Connacht; (5) the kingdom of Aileach, the Fochla, or the Northern Ui Néill; (6) the kingdom of Ulaidh or the lesser Ulster; (7) the kingdom of Oriel.
In Munster, a sort of partitioning or appropriation was effected by the ruling Eoghanacht dynasty, similar to what has been described as taking place in Connacht and Meath. At the head of all was the Eoghanacht of Cashel. Cashel was surrounded by a zone of tributary States, whose rulers were not of the Eoghanacht lineage. Westward of these was a belt of Eoghanacht States extending across Munster from the Shannon to the southern coast. These comprised the Ui Fidhgheinte in County Limerick, the Eoghanacht of Aine, in the middle, and the Ui Liatháin to the south in parts of Cork and Waterford counties. There was another Eoghanacht kingdom in the region of Bandon. Finally there was the Eoghanacht of Loch Léin in the region of Killarney, called also the Eoghanacht of West Munster. I have already shown reason to think that the Eoghanachta represented a relatively late immigration from Gaul; that their original settlement was probably in the west of County Waterford; and that their conquest of south-western Leinster and occupation of Cashel may have taken place about the beginning of the fifth century. I have no means of fixing the date of their occupation of other parts of Munster, but these settlements are not likely to have been later than the fifth century.
In like manner, we find located in various parts of Leinster the septs that branch out from the royal line. I shall not cumber your attention with the details, which can be found in O'Donovan's notes to the Book of Rights. A much larger proportion of Leinster was appropriated in this way than of any of the other chief kingdoms, except Oriel. Oriel, being the main part of Ulster conquered by the Connacht-Meath princes in the fourth century, was treated entirely as a land of conquest, no portion of it remaining under the rule of its earlier dynasts.
In the case of Leinster, the relative lateness of these appropriations is proved by one fact. The septs that became possessed of territories in this way all belonged to the old ruling house of South Leinster, but the territories appropriated to them are very largely situate within the bounds of the old kingdom of North Leinster. Hence the resettlement of these territories took place after the extinction of the North Leinster kingdom and the unification of what remained under the South Leinster dynasty. This shows that the process belongs to the same period in Leinster as in Connacht, and Meath, and Munster.
Though the annexation of Tara and Bregia was a fully accomplished fact long before St. Patrick's time, and though in his time the monarchy of Connacht origin was securely seated in Tara, the annals, whose details of history begin with St. Patrick, show that the claim to their northern territories was not yet relinquished by the Leinstermen. Time after time they invaded the lost land, and battle after battle was fought by them on its borders and even far within its borders. This continued struggle to recover possession is perhaps most clearly seen in a list of the battles from the year 432 onward—before that year we have no details.
A.D. 452. A great slaughter of the Leinstermen.
A.D. 453. The Leinstermen defeated in battle by Loeguire son of Niall [i.e. by the King of Tara].
A.D. 458. The battle of Áth Dara. Loeguire, king of Tara, is defeated by the Leinstermen and taken prisoner.
A.D. 464. Leinstermen win the battle of Ard Corann.
A.D. 473. Ailill Molt defeats the Leinstermen at Brí Éile. Ailill was king of Tara at this time. Brí Éile was in the kingdom of Meath.
A.D. 474. The Leinstermen defeat Ailill Molt at Dumha Aichir.
A.D. 486. Battle of Granard. Finchath, a Leinster king, was defeated and slain. The sept of Cairbre, son of Niall of the Nine Hostages, was victorious. This sept held territory around Granard, and they were therefore resisting invasion by the Leinster king.
A.D. 487. Battle of Gráine in Kildare. Muirchertach, king of the Northern Ui Néill, defeats the Leinstermen.
A.D. 494. Battle of Tailltiu (=Teltown, near Navan). The Leinstermen are defeated by the sept of Cairbre, son of Niall.
A.D. 498. Battle of Inne Mór in Kildare. Leinstermen defeated by Muirchertach, king of the Northern Ui Néill.
A.D. 499. Battle of Slemain, in Westmeath. Leinstermen defeated by the sept of Cairbre, son of Niall.
A.D. 501. Battle of Cenn Ailbe in Kildare. Leinstermen defeated by the sept of Cairbre.
A.D. 503. Battle of Druim Lochmhuidhe. The Ui Néill defeated by the Leinstermen.
A.D. 510. Battle of Fremu, in Westmeath. The Leinstermen are victorious over the sept of Fiacha, son of Niall.
A.D. 517. Battle of Druim Derge. The Leinstermen are defeated by the sept of Fiacha. This was regarded as the final and decisive battle, which forced the Leinstermen to relinquish their attempts to recover the lost territory in Meath. "By it the plain of Meath was lost and won," says the poet-historian Cenn Faelad in the following century.