When I listen to Celtic music on or before St. Patrick's Day each year, "Fancy Waistcoat" by Niamh Parsons & The Loose Connections always makes my playlist. Parsons sings it with the right amount of carefree joy, which is partially why it works so well. The other essential ingredient is its blend of Celtic and Spanish styles. Not every Celtic tune can pull off such catchy guitar, keyboard, and accordion backing.
I'm still looking for the right tarantella to segue with it, but until I find it, I'm content to hear "Fancy Waistcoat" as a standalone tune.