|Cahill||Very numerous: all provinces but least in Ulster. Ir. Ó Cathail, a very popular personal name in early Ireland (see entry). There were septs in Clare, Tipperary and Kerry. It is one of the earliest surnames on record. IF & SGG.|
|MacCahill||fairly rare: Donegal-Tyrone-Cavan. Ir. Mac Cathail from first name Cathal meaning "strong in battle". May also stand for Mac Cathmhaoil (battle chief).|
Parishes where McCahill and a second surname are found together
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