"The only two "traditional" instruments in our music that I know of are the harp and the bodhran. In both cases there is discussion about origin, but no dispute that they have the longest history in the stories of our people.

I think you'd find a fair bit of dispute about the bodhran, and it'll probably show up in this thread. The idea of stretching a bit of hide across a frame and hitting it is old enough - I suspect it goes back to Cro-Magnon times or before. (Maybe they picked it up from the Neanderthalers...)

In Ireland it was used on particular occasions in particular places, notably as part of the "hunting the wren" tradition. But as an instrument played together with other instruments to provide the percusssion, as I understand it, that's no more than 40 years old, and was a Sean O Riada innovation.