No adverts at all right now. I was looking forward to all those gorgeous dark ladies wearing nothing but handbags.

Fiddler's Companion gives you a pretty one-eyed perspective. The vast majority of the sources cited are Irish or Irish-American, often thoroughly obscuring a tune's longer and well-established origin and development in England or Scotland. And they never, ever correct anything. TheSession works much better since you can be sure someone will be along eventually with a post that sets the history straight.

http://abcnotation.com has a graphical display of related tunes when you look at one in particular. I'm not sure how the relatedness is calculated.