In the beginning of the 4th century, three warlike brothers, known as the Three Collas, made a conquest of a great part of Ulster, which they wrested from the Ulaid.
The Kingdom of Airgíalla came to an end in 1585 when Rossa Buidhe Mac Mathghamhna agreed to surrender and regrant his territories to the English Crown in Ireland, with his territory becoming County Monaghan in the Tudor Kingdom of Ireland.