c- in Irish clann "offspring"

Question

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:

1. 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?

2. 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?

Answer 1

You have to a large degree answered the question yourself, really: clann is indeed a very early loan word.

Like Common Celtic, Common Insular Celtic had no /p/, but it had /kʷ/.

The Brythonic branch changed /kʷ/ to /p/ (keeping the labiality, losing the velarity) quite early on, whereas in the Goidelic branch, /kʷ/ merged with /k/ (retaining the velarity, losing the labiality) somewhat later on.

In the earliest Ogham inscriptions, there are still separate letters for /k/ and /kʷ/, so we know that the change took place fairly close to the ‘proper’ attested stage of Early Irish. Sadly, we don't have any direct evidence that dates the Brythonic change, at least not that I know of. But unless I'm misremembering (and I don't have access to my works on Brythonic sound change at the moment), it can be positively shown to have predated certain other changes that definitely happened quite early on.

At the time when clann was borrowed, Brythonic /kʷ/ => /p/ had already taken place, but Goidelic /kʷ/ => /k/ had not. In other words, Brythonic *pland was borrowed with the closest equivalent available (the only unvoiced labial plosive in the language), as *kʷland, which later lost its labiality and became kland (orthographically since Old Irish cland, later on clann).

Once /kʷ/ was lost in Irish, however, there was no good, existing unvoiced plosive to use as a substitute for /p/, and it was around that time that they started using this hitherto foreign phoneme in loan words, reintroducing the long-lost /p/ back into the Irish language. This was still fairly early on, of course, predating the earliest ‘proper’ Old Irish sources (the Milano and Würzburg Glosses)—early enough, also, that this new /p/ became a natural part of the system of initial mutations on a par with /k/ and /t/, all three being subject to both lenition and nasalisation.

I am certain there are other words (either directly from Latin or through the medium of Brythonic) that show a similar route, but I cannot think of any off the top of my head at the moment.

Edit: Not true, there is one other well-known example I can think of: the Irish word for Easter, Cáisc, is derived via Latin pascha and Greek πάσχα ultimately from Hebrew (see etymonline for details). That shows the same development.