The exact meaning of irony is long debated, but it usually means when someone says something, but actually means exactly the opposite.
"I'll Be Home For Christmas" is a wistful song about someone who will probably not make it home for Christmas.
"I Saw Mommy Kissing Santa Claus" could, also, be considered ironic, in that a child is telling of a scandalous event, while he, himself, never realizes what he actually saw.
