"Ann am muga."
Translation:In a mug.
It's a historic thing.
The word ann on its own means in and always has. However, an/am, as well as meaning the, used to also mean "in".
So am baile would have meant both the town and in a town.
Obviously this could cause confusion so people started saying ann an (literally in in). Ann an has stuck, but an on its own meaning "in" hasn't.