Sean-nos or “old style’ of singing as Gaelic suggests has been part of Irish music for ages. I would pull reference to singers like Iarla O’ Lionaird and,Mairéad Ní Mhaonaigh of Altan among the many. According to shamrockirishmusic.com, sean-nos can be:
1. A bare voice (not ‘sweet’, with a certain ‘natural fierceness’).
2. No vibrato.
3. No dynamic. (loud/soft)
4. Emotion is expressed through the use of vocal ornamentation, which varies from singer to singer.
5. Free, non-metronomic rhythm used by the singer.
6. The meaning of the words dictates singing from the heart, with ‘soul’. (Without dynamic – see above).
7. Often there is an emphasis on the consonants l, m, n, r to facilitate the free rhythmic pulse and to create a drone effect.
8. Occasional nasalisation.
9. Music takes precedent over the lyric.
10. Often extra meaningless syllables are introduced, e.g., “Thug (a) me”.
11. The use of the glottal stop/dramatic pause.
12. It’s unaccompanied.
13. The melody varies from one verse to the next, and from one performance to the next. This is often referred to as the ‘variation principle’.
14. And last but not least, the singing is in the Irish language.
The whole article can be found here.