I found something relevant to the question whether to read "a himirge" or "a imirge" in the two versions A and E1 of Cath Maige Rath. In both texts "a" "his" is followed by
A: a hinchind dermait du beim a cind Chindfaelad "his brain of forgetfulness being beaten out of the head of CF"
E1: dara heis i neirind "behind him in Ireland"
This suggests that "h" could be written (mistakingly?) even after "a" as 3rd sg. masc. possessive pronoun.