"Ar Hyd y Nos is a Welsh folksong sung to a tune that was first recorded in Edward Jones' Musical and Poetical Relics of the Welsh Bards (1784). The Welsh lyrics were written by John Ceiriog Hughes, and has been translated into several languages, including English and Breton. The melody was used by John Gay in The Beggar's Opera. It is also used in the hymn "Go My Children With My Blessing." The song is highly popular with traditional Welsh male voice choirs, and is sung by them at festivals in Wales and around the world. The song is also sometimes considered a Christmas carol, and as such has been covered by numerous artists on Christmas albums." - Wikipedia
I arranged this a while ago, to play at a funeral. Nothing complicated, the melody simply passing between instruments with variations in the accompaniment. The violin and cello parts are around Grade 3 standard, but the piano is a bit more tricky. It's quite long, but you can always use just a few of the variations or skip some out.
It was produced with Finale 2009, which means it's not accessible to everybody, but there's a Finale Reader programme available free of charge, and I've included MusicXML and MIDI exports, which can be used in other programmes.