MacKennan/McClellan? Are they the same?

Hi! Can anyone tell me if McClellan is a variant of MacKeenan? I have a marriage record where everything fits but the maiden name is listed as Keenan, not McClellan (from the d.c.). I saw a translation that says they both mean :Mac Gille Fhaolain (Scottish) and Mac Giolla Fhaoláin (Irish) ‘son of the servant of (Saint) Faolán’. Can anyone confirm this for me, because if true, it's a big link for me! Of course, then I have to find out why there are two surnames on two certificates, but that's another step. I'd be so grateful if anyone could advise me, and thanks!!!Merry Christmas!

Kitty