Irish clann "plant; offspring; child" (the source of English clan) is borrowed from Welsh plant with the same meanings, which is itself a borrowing of Latin planta. Why did Irish change the initial p into c?
PIE p was lost in Celtic, so the explanation I've seen given is that Irish did not have initial p at the time of the borrowing. But this leads to other questions:
There are in fact numerous Irish words beginning p-. These, as far as I can tell, are mostly loanwords. Why the p in these words but not in clann? Do the loanwords in p date from a later period, and if so when did Irish acquire a p and begin to retain p in loanwords?
Why the specific change to c? Other than being voiceless stops, the two sounds are not particularly close; one might have expected the p to be lost completely, or maybe to be borrowed as b. A possible factor is that there are numerous cognate sets where Irish c corresponds to Welsh p (from PIE * kw), e.g. cethair : pedwar 'four'. If speakers were aware of these correspondences, this could explain the choice of p. Is there any evidence that this might have been the case?
Finally, are there other Irish loanwords from Welsh (or other languages) which show substitution of c for p?