Faerie Queen:-
Conán is quite correct in his observations.
The tune is called "Sidhe Beag, Sidhe Mór". The question of pronouncing "BH" or "MH" doesn't even apply here, so I don't see the point of the observation.
The word "Sidhe" in the title does not mean a "Fairy" it means "a fairy mound". There is a difference.
The construction you cite as meaning "it is little/it is big" does not exist in Irish. There are two basic ways of saying "It is small / it is big" :-
1. Tá sé (sí if the Subject is Feminine Singular)beag / tá sé mór."
2. 'S é (í if Feminine Singular) atá beag / 'S í atá mór. This would be used for emphasis.
3. There is another rather less usual construction.:-
Is beag é (í if Feminine Singular)/ Is mór é.
That's it.