At the period when Ptolemy wrote, the Nagnatae were the inhabitants of the whole of the county MAYO, with the exception of a small portion of its southern extremity, into which the Auterii, who were settled in the northwest of Galway, had penetrated. The ancient chronicles state that at the commencement of the 4th century the whole of Connaught was taken from the Firdomnians, a branch of the Firbolgs, who had held it till that time under the Milesians. Later, M. Vaugondy's map of ancient Connaught furnishes the following name of the territories which composed it, and their respective baronies; Irrosdomnion, being the barony of Erris; Calrigiamuighemurisk-in-Amalgaid, and Hy Fiachra Aidhne, the barony of Tyrawley; Coranne, the barony of Gallen; Con-macne-Quiltola, the baronies of Clanmorris and Kilmain; Kierrige de Lough Nairn, the barony of Costello; Hymalia or Umaille, the barony of Murrisk.

The Airthir had split into the main septs by the 8th century, the Uí Nialláin, the Uí Bressail, and the Uí Echdach. About the 10th century, some of the major groups in the modern co. Armagh region included the Uí Méith, the Uí Nialláin, and the Uí Bresail. The oldest on record is Flaithbheartach Ua'h-Anluain, Lord of Uí -Niallain, whose murder in the yeare 983 A.D. is recorded in the Annals of the Four Masters. In the south east of Airghialla lay the Kingdom of Airthir, meaning Eastern (i.e. Eastern Airghialla). This was the stronghold of the Ua'h-Anluain. Airthir covered a large tract of what is now southern County Armagh, spilling over slightly into Counties Down, Louth and Monaghan. The Kingdom of Airthir was also known as Ind-tÁirthir, Oirthir and later Orior.

In the 12th century, the Irish Annals note Ua Ruadhacain (O'Rogan) as chiefs of Airthir, an area which may have been more restricted in size. The Ua Ruadacháin were noted as chiefs of Uí Eachach (Echdach), tributary to the O'Hanlons at the time, in Smith's The English in Louth 1170-1330. O'Dugan (Poems) mentions the two kings over Oirtheara as O hIr, and O hAnlauin. The Kingdom of Airghialla was broken up by the Saxon-Gall (Anglo-Norman) advance into Ireland in the late twelfth and early thirteenth century. The Annals of Ulster record the Norman advance and a rare victory for the Airghialla: U1211.1 The Foreigners came to Narrow-Water, until Aedh Ua Neill assembled Cenel Conaill and Cenel Eogain and the Airghialla, so that they [the Foreigners] were killed by him. The group that settled in North Louth were known as the Conaille Muirtheimhne and took their name from Conaill Carnagh, legendary chief of the Red Branch Knights of Ulster. Their land now forms upper and lower Dundalk.

Dundalk town's coat of arms still has an ermine boar representing the O'Hanlons supporting a shield with the De Verdon coat of arms (representing the Norman conquerors). One of the lasting features they left behind is the Proleek Dolmen at Ballymascanlon, on the northern side of Dundalk. It takes its name from Dún Dealgan, Dalga’s fort home closely associated with the famous mythical warrior Cúchulainn and was granted its charter in 1189. It is sited on the lowest bridging point of the Castletown River. The Castletown River rises close to Newtownhamilton Co. Armagh and is known as the Creggan River in its upper reaches. Its two main tributaries are the Kilcurry and Falmore rivers, both of which contain stocks of wild brown trout. Dún Dealgan (“Dealgan's Fort”) seaport, urban district, and administrative centre of County Louth, Ireland. It lies near the mouth of the Castletown River on Dundalk Bay. It received charters from King John about 1200 and from other monarchs.

When Edmond Mortimer arrived in Ireland in 1380, The Annals of The Four Masters records that the O'Hanlons as Lords of Orior were considered to be amongst the mightiest lords of Ireland, i.e. The Roydamna of Ireland: Mortimer came to Ireland with great powers, as Lord Justice; whereupon the Irish nobility repaired to [pay their court to] him, and among others the Roydamna of Ireland, i.e. Niall Ó Neill, O'Hanlon, O'Farrell, O'Reilly, O'Molloy, Mageoghegan, and the Sinnach Fox, with many other nobles.