I did some research after reading a post from a Gerard Sisk..and found the following info, all from churchrecords irisgeneology.ie

This connects my Mahoney/Buckley family to the Sisk family:
Hi Gerard,
I found a clear link to your Richard Sisk and his brother Denis from Ballinhassig. My great grandmother, Mary Sisk of Abby, Ballinhassig, was born 3 Nov 1878 to John Sisk and Julia Hurly..sponsors Peter Warner and Mary McCarthy.

Richard was born on 2 Feb 1865 to same parents, sponsors Denis Hurly and Hannah McCarthy

Denis was b 4 Mar 1877, also lived at the Abby, Ballinhassig, and sponsors at baptism were John Hurly and Ellen Connor.

other siblings are:
Michael Sisk, b 25 Aug 1875, sponsors Robert Wardner (probably Warner), and Kate Whelton

Patrick Sisk, b 31 May 1867, sponsors Patrick Hurly and Ellen Fenton

Kate Sisk b 7 Jan 1862 sponsors Daniel Conway and Ellen Hurley.

The parents were married in same place: The Abby, Ballinhassig on 9 Feb 1861 John Sisk married Julia Hurly, witness for husband was Timothy Sexton and Norry Hurly for wife.

Note...Timothy Sexton is very likely a brother in law, because on very same page of parish register, we see him marrying an Ellen Sisk They were marries on 20 Feb 1859, witnesses Timothy Sisk for husband and Thomas Punch (sp?) for wife.

These people are all related to me because Mary Sisk of 1878 married James Buckley, and they had my grandmother Margaret Buckley Mahoney, born in 1910.