I play the same types of recorders as Moira plays in addition to lots of strings. I get to celtic music at times and play it often with those who specialize in it much more than I do. They might have some complaints about my playing of the instrument, but they have always been very supportive of the use of the recorder with their celtic music. Duets with whistles or flutes and recorders (using the same voicings as Moira suggested) have been very well received. Go ahead and play the recorders with celtic tunes and folks. If you're overly concerned about reaction from hard-core celts, just keep an eye and an ear open as you play so that you will know what they think about your use of the instrument. There are those in old-time and bluegrass who insist that their music should only be played with certain combinations of instruments. I let them play their strict ways, and I play with others unless there's no other music around -- and then I play as close as I can to their ideas while trying to make just a little trouble by doing something out of their set ways every so often. I don't think I've made any enemies that way, but I have sometimes felt the circles closing me out.