From Wiktionary, the free dictionary
From the common occurrence in such songs of an exhortation beginning "come all you".
come-all-you (plural come-all-yous)
- (dialectal, British, Ireland) A traditional folk or country song.
2006, Patrick McCabe, Winterwood, Bloomsbury, published 2007, page 3:He slapped his thighs and whooped and catcalled, encouraging anyone who knew it to join in the ‘traditional come-all-you’!