We should be careful not to attempt to delineate too narrowly what is or isn't 'sean-nós. There are no rules about it, only time-honoured conventions. The term 'Sean-nós' itself is just Irish for 'old style' or 'old custom'. Thus we have sean-nós dancing as well as singing. When applied to singing in Irish it means sung in the traditional style. Styles do vary from district to district but they are all correctly called 'sean-nós'( notice the long 'oh') . If the song is an English-language song then I'd say just announce it as a song 'in the traditional Irish style'.