I second Canadee-i-o by Nic Jones.

Not acoustic but definitely folk: Go from my Window by Liza Carthy

Not Folk and definitely not acoustic: Hotel California

Both of these set up the atmosphere of the song.

Not guitar but definitely folk: The Holly and the Ivy by Magpie Lane. The intro is on fiddle and is the tune played through in a way that shows its dance origin. I can imagine a Morris side dancing to it.