It is hard to categorise a lot of folk songs as they tend to move from one country to another, and not just within the United Kingdom. There are versions of many of these songs in America and all over Europe. So these divisions should not be taken too seriously.

The most popular folk songs these days seem to come from Ireland, so I'll start with songs of Irish origin, then English, then Scottish.

Songs from Ireland

Songs from England

Songs from Scotland

Songs from Wales

Of course the Child Ballads are also a very important set of folk songs from England and Scotland.