Most Co. Kilkenny RC parishes have records back to the early 1800s including all four parishes in Kilkenny town. Castlecomer records start in 1812.
I searched the subscription site Roots Ireland and found a July 8 1811 marriage between Patrick Brennan and a Mary Delaney. The marriage was in the Muckalee RC church. Any possibility that this is the marriage you are looking for?