Like the h before É and the b before P in "i bPoblacht na hÉireann", others like "an nGaeilge", "na nAirí" "i dTuaisceart Éireann" and so on, what do they stand for? I can look up every single word easily, but I can't find these prefixes.
As you'll find out from the link that CapnPrep posted, they aren't prefixes as such but are an indication of the initial consonant sound changing. So in your example of "i bPoblacht na n'Éireann", the word "poblacht" is usually pronounced with a /p/ sound but as the word follows the word "i" which triggers lenition, then it's pronounced as a /b/ sound. The spelling reflects this.