1. Rambling can mean several things, but in this song, it means "wandering" or moving aimlessly. 2. "There's none so rare" suggests that these people are "one of a kind" (special and unique). 3. "On the tear" is an idiom and it means that you're behaving energetically. It often describes someone who is engaging in a series of deviant acts (drinking, using drugs, being abusive to people).

the apostrophe indicates a letter is missing. The letter is g - so look up 'rambling'. It means walking around, or wandering, or possibly hiking. There's none so rare is more correctly there are none so rare - meaning that nobody is as rare - or that they are unique, or special. I have no idea what 'tear' is. I just looked it up: http://www.tripadvisor.com/Travel-g186591-c2351/Ireland:Irish.Slang.html says that it means 'out drinking'.