"Killahy", a parish, in the barony of Knocktopher, county of Kilkenny, and province of Leinster, 6 miles (S.) from Knocktopher, on the road from Waterford to Kilkenny; containing 712 inhabitants, and comprising 2670 statute acres, It is a vicarage, in the diocese of Ossory, forming part of the union of Kilbeacon, or Rosinan; the rectory is impropriate in the Ladies Fitzpatrick. In the R. C. divisions it forms part of the union of Kilmacow. About 30 children are educated in a private school.
[From A Topographical Dictionary of Ireland (1837)]
Note: There is another Killahy parish in the barony of Crannagh, next to the civil parish of Tullaroan, and in the R.C. parish of Urlingford.