It has taken me weeks of cogitation and reflection to work out the words to describe my impression, of sean-nós sung by Máire Ní Scolaí. Here is an attempt.

As I have posted on a different thread, the sound of Máire Ní Scolaí's singing voice exposes the technique of breath control and support with which she sustains long, ornamented, nearly instrumental melodic passages. I hear her classical singing technique especially in that tone, and in the breath support exposed in that tone. She has the breath control, what the Italians prefer to call "appoggio," of a continental European singer -- a great one, I would have to say; her singing is in fact a master class in how to let the voice ride a column of air, on the breath. Opera singers could profit from listening to her recordings.

What is the effect of this sort of sound production in traditional unaccompanied singing in Gaelic?

If I were to compare it to the singing of, let us say, Liam Clancy, himself a seasoned artist and performer, this is how I would phrase it: Liam's voice has the grain of wood, solid yet living wood; while Máire Ní Scolaí's recorded voice sounds like it is carved out of marble and polished until it shines. Necessarily, one sounds more organic and alive than the other. Some listeners have a horror of marble statues in their music rooms; some desire little else. I personally would not be without either one.