The heartland of Dál nAraidi was the Six-mile River valley (known then as Mag Linne, which has become modern Moylinny). Except for their ninth century extension across the river Lagan, Dál Fiatach were usually south of the river. In other words, the lands between the Lagan valley and Crumlin, and between the mountains and Lough Neagh, were borderlands between these powerful tribes. Not surprisingly, the two locatable battles between them took place on or near the Lagan, in A.D. 668 at Fertas, the ford that was much later to become the city of Belfast and in A.D. 1025 at Ard Achaidh, which seems to be modern Derriaghy (see, for instance, Reeves, 1847, 46, note g). Early writings describe the area and its people sometimes as belonging to Dál nAraidi and sometimes as belonging to Dál Fiatach, and this is precisely what we would expect. In the medival period much of this area was known as Killultagh, Irish Coill Ultach - the 'wood of the Ulaid'. Sixteenth century descriptions show it to have been very heavily wooded. The name survives in the townland of Derrykillultagh and Killultagh House in that townland. It is known that some baronies reflect earlier tribal areas and it is interesting that the borderland we have defined is fairly exactly the area of the barony of Upper Masserene. In the borderland area a tribe known as the Dál mBuinne (pronounced Dawl Mwinner).

Although the kings of Dál Fiatach were usually located south of the Lagan in modern Co. Down, there were occasions in which they had their headquarters north of the river. For instance, the ninth century king Matudán son of Muiredach, who was also king of the federation, appears to have had his capital at of near the Cave Hill, north of Belfast, which is also known as Benn Madighan (the `peak of Matudán').

Their name, and possibly their original territory, is preserved in the medieval rural deanery of Dalboyn-which was roughly bounded by Aghagallon, Blaris, Drumbo, Derryiaghy, Tullyrusk and Glenavy (Reeves, 1847, 44 and 172). This is the borderland area as we have defined it, with a small extension south of the Lagan. The ethnic background of Dál mBuinne is difficult to establish, some sources implying that they were Ulaid and others that they were Cruthin.

Many of the early kings of Ulster through the centuries were of the Dál n-Araidhe and the Dál Fiatach clans of counties Armagh, Down and Antrim.