In fact, “cara” and “carr” have totally different vowels in them, respectively represented by /a(:)/ and /É‘Ë/ in IPA. The former almost resembles /Ã¦/, while the latter is not too far away from long “o”, so it would be misleading to regard them as allophones.
The /É‘Ë/ sound is usually spelt “á” as in “bád”, but also appears in words like “carr”, “mall”, “ball” where “a” is followed by a double consonant. On the other hand, “cara”, “dara”, “marbh”, “ach” and “nach” all have the vowel /a(:)/.
As regards the issue of allophones, you might perhaps say that /a:/ is an allophone of /a/, since the length of this particular vowel may vary depending on the dialect area and speaker. Indeed, in connectives and particles like “ach” and “nach”, the /a:/ sound can even turn into /É™/ when not emphasised.
What I’ve said only applies to Connemara.