"I like Christmas."
Translation:Is maith liom an Nollaig.
Yes, Nollaig by itself (or Mí na Nollag) would be December (note, the form without the < i > is the genitive!), whereas an Nollaig is Christmas.
A very late reply, but in case someone is still wondering, this is what Wiktionary has to say:
From Old Irish Notlaic, from Latin nātālīcia (“a birthday party”), with specific reference to the birth of Christ, from nātālis (“natal”), from nātus (“born”). Compare Welsh Nadolig, Breton Nedeleg and French Noël.