I think I know what Jim's trying to say, but Irish traditional music has little to do with improvisation as the term would be understood in other genres. Since maintenance of a tune's melody and its rhythm lie at the core any improvisation is limited to subtle variations around said melodic elements via means such triplets, rolls or the piper's cran.

Indeed those who do improvise around a tune's melody are often regarded as being outside or divorced from the tradition. The late fiddler Tommy Potts certainly fell into such a categorisation and Martin Hayes has also fallen foul of this criticism. Personally, I regard such suggestion as unjust in both cases.