An Cháisc is derived from Latin Pascha (from Greek Πάσχα Pascha) and shows a sound change from p to c which sometimes occurs in loanwords into Old Irish.
I don't think that's quite right - Q-Celtic didn't change p to c.
Proto-Celtic is thought to have changed Proto-Indo-European p to an "f"-like sound, which was deleted, like how Latin faba became Spanish haba, so the ancestor of the surviving Celtic languages had no p sound.
So PIE ph₂tḗr >> PC ɸatīr >> Old Irish athir >> Modern Irish athair
P-Celtic languages got the p sound back by changing kʷ and gʷ to p and b. This leads to: PC makʷos >> Welsh mab (p is lenited to b in Welsh).
Q-Celtic merged kʷ and gʷ with k and g. makʷos would be expected to give -mach-, as c is lenited to ch in Irish. This means mac would come from a local makkʷos pronunciation. It also means Q-Celtic still had no p, so couldn't have changed p to c.
Early Irish that was still close to early Welsh, so speakers might have noticed Welsh p matched with their k. So Pasg was reshaped as Cáisc. It doesn't look like you can infer a general rule from this though - Padrig became Pádraig rather than Cádraig.
Even today, nearly all words with beginning with p in Irish are loanwords.
Thanks for correcting me. I didn’t express myself clearly enough and I’ve amended the post accordingly.
Yes, Indo-European p did not become Old Irish c. The p was often lost, as in the case of athair or mutated, I think, into a /b/ or /x/.
Where Old Irish borrowed words from Latin (sometimes via Welsh, or I guess Brittonic at the time) we do sometimes see a p > c change. I think this may be the case with Latin presbyter ‘priest’ > Old Irish cruimther ‘priest’ or planta ‘shoot, plant’ > cland ‘offspring’.
Not all loanwords show such a sound change e.g. Old Irish penn ‘pen’ from Latin penna ‘feather’. Patrick is an interesting case because Irish adopted the p spelling but I’m pretty sure you get something like Cothrige in some early texts. I’d have to double-check.
No, it was correct. Often loanwords into OI would have their /p/s turned into a /k/. For instance, corcra (purple), cothraig (older form of 'patrick'), and a bunch of others were borrowed like this.
Yes, Fingolfin clarified that he meant that in his second comment, and edited his first one accordingly. Thanks though!
"Is maith liom" is usually reserved for "I like", and stronger versions like "Is breá liom" and "Is aoibhinn liom" are used for "I love".