Look for the lenition or not following. His: Tá sé in a chónaí (He is in his living) and Her: Tá sí in a cónaí (She is in her living).
In your example Is fearr liom a bialann--her restaurant
And 'a' can also mean their, which causes eclipsis. So varying the example sentence:
- Is fearr liom a bhialann. (I prefer his restaurant.)
- Is fearr liom a bialann. (I prefer her restaurant.)
- Is fearr liom a mbialann. (I prefer their restaurant.)