Families of the name arose separately in different parts of Ireland, in Kerry, Galway, Tipperary and Clare. Originally the Galway family, located in the old diocese of Kilmacduagh near the Clare border, were most prominent, but their position was usurped by the O'Shaughnessys, and they declined. The southern families flourished, and the name is now most common in counties Cork, Kerry and Tipperary, while it is relatively infrequent in its other original homes. The arms illustrated are those of the Munster Cahills.
With 147 births of the name in 1890, it was ranked 134th most common in Ireland. It now stands at 142nd.
The geographical spread of families of the name is reflected in the 45 placenames which incorporate Cahill. These placenames appear in Clare (4), Tipperary (6, including 3 Ballycahills), Galway (8), Cork (2) and Kilkenny (1)
.A very early bearer of the name, whom the Annals record as having been martyred in 938 was Flann O?Cahill.
Another churchman was Father Daniel Cahill (1796-1864) was well known in his day as a teacher, journalist and lecturer.