The baptism records for Kilclooney Church of Ireland only start in 1832, marriage sin 1835 and burials in 1837. Consequently, if William was baptised in that parish c 1817, there won’t be baptism records to find. Likewise if that’s where his parents married, you won’t find that record either. You could try searching the burial records to see if you can find a John Warnock and a female who might be his wife. The only copy of the records is still held by the Minister and so you would need to contact him/her to ask for a search. There’s likely to be a fee for the service. The Church of Ireland used to charge £19 (sterling) an hour. I don’t know if that’s still the current rate but I am sure the Minister will let you know.
From the 1851 marriage (on rootsireland), I can see that William Warnock was living in Cordrummond when he married. He may have been living there with his parents (assuming both were still alive). Both he and his father were labourers. Labourers tended to move around a bit to follow the available work and can be tricky to trace. There’s no Warnock household in Cordrummond in 1864 (Griffiths Valuation). No John Warnock anywhere in Kilclooney in 1864, but of course he might have died by that date.