So, a is his, her and their bu:
a (his) lenites the consonant, no changes with vowels (a bhuachaill, a úll)
a (her) no changes with consonants, but h before vowels (a buachaill, a húll)
and a (their) eclipses the consonant, (what happens with vowels in this case?) (a mbuachaill, a...?)
Is that right? God, that's confusing, but I'm loving it!
To quote galaxyrocker from this page:
Because some possessive pronouns lenite, and some eclipse.
mo, do, a (his) lenite
a (hers) adds "h" before a vowel
ár, bhur, and a (theirs) eclipse