The h in phortán is called a séimhiú*, the process is called "lenition". Lenition only applies to consonants (not all consonants can be lenited.
You can't lenite a vowel. The h in a húll is not a séimhiú, it's a h-prefix.
a meaning his lenites nouns that start with a consonant. It doesn't do anything to words that start with a vowel.
a meaning "her" doesn't do anything to nouns that start with a consonant. It causes a h-prefix for words that start with a vowel.