The embellishment you ask about, in the Irish tradition, is drawn, in its purest sense, from something deep within us. You mentioned Joe Heaney and rightly so. His style is representative of the Connemara sean-nos style which is very much alive in those parts today. The embelishments heard in his voice could probably be traced to the origins of the Celtic race, and can also be heard in the singing of those celts who moved east ie. India. However, when all is said and done the individual singer should develop his/her own feeling for when and where and to what extent they use such embelishment. I heard one sean-nos singer liken it to blowing up a balloon....you need a certain amount to get it right but too much and it will burst ie destroy the song.