Is this construction, "...in the month of X," used of all months or just April?
Could ritheann siad in Aibreán also be used? Or is it only possible to say “in [the] month of […]”?
In 'mí Aibreáin', 'Aibreáin' means 'of April'. This is because 'Aibreáin' is the genitive case form of 'aibréan'.
There's a difference in pronunciation and the 'i' isn't just decoration: it signals that the 'n' is 'slender', so it has a pronunciation not unlike 'ñ' in Spanish.