The kingdom of Brega included the Hill of Tara, the site where the High King of Ireland was proclaimed.
It extended northwards across the River Boyne to include Sliabh Breagha the line of hills in southern County Louth.
In later centuries Brega was threatened by the rise of the Viking Kingdom of Dublin and came under the suzerainty of the kings of Mide.
In the divisions of that kingdom in the twelfth century parts of Brega, or East Mide, came under the control of Tigernán Ua Ruairc of Breifne and Diarmaid mac Murchadha of the Laighin.
The Uí Chonaing had earlier been settled around Tailtiu and Ráith Airthir in the valley of the Blackwater; that district was left to another branch of Síl nÁeda Sláne, Síl nDlúthaig upon the conquest of the Ciannachta Breg during the reign of Cináed mac Írgalaig.