Look into any form of traditional music from the wider European continent and you will find accordions. Scotland has a strong accordion tradition easily adapting to their snappy melodies and capable of imitating the sounds coming from the bagpipes, they are also just as versatile with Irish tunes. They come in all manor of sizes and a great instrument for someone with some previous experience on the piano to jump into Celtic music.
Who you should listen to: Phil Cunningham (Scotish), Martin Tourish (Irish)