The verb is cabhraigh, so it will always begin with either cabhr-, chabhr-, or gcabhr-, depending upn the conjugation. Cabhraímid le X means “We help (with) X” — the subject “we” is fused into the verb. The sentence’s object is na buachaillí ; the preposition le changes to leis before an article. (It isn’t the prepositional pronoun leis.)
Imím ar maidin leis an mbuachaill agus fillim leis an gcailín
Your question isn't clear - I'm guessing that you're actually asking why it's leis na buachaillí versus leis an mbuachaill?.
Eclipsis occurs after certain simple prepositions (such as ag, ar, le, etc) and the singular definite article an. It doesn't apply to indefinite or to plural nouns.