There's a fundamental difference between style and content. Speaking puristically (is that a word?), sean nós MUST be sung in Irish. Irish language songs are basically emotive, with little in the way of narrative (and I am being very general here), as opposed to English language songs, which tend to be more narrative and balladic. Thus, the style of sean nós is very much about the emotion of the piece, and this is done through vocal technique and localised stylings. Joe Heaney is one of the finest examples of the genre - not what everyone would call a wonderful singer in terms of his actual voice, but in terms of technique, quite a class apart. English language songs can be a sung in a traditional "style", sure, but the proper sean nós exists on a deeper level than vocal purity and/or dexterity. Someone like Liam Weldon has been described as singing in a sean nós style, which is accurate in terms of emotional intensity. I've been described as having a sean nós influence, but this was probably by someone who was unfamiliar with Ulster style, and needed a convenient adjective.
peace and love a.w.