Thanks to both of you, and sorry for the delay in replying.

Point taken about the fact that singers also ornament. My main difficulty is with the fact that sometimes - espêcially given the free rhythm of sean-nós - the overload of ornamentation upsets the balance of the melody, so that what in the lyrics is an unstressed syllable ends up sounding too heavy.

I'm not saying that you have to know the lyrics of a song off by heart to be able to play it properly, but they're still a useful guide.

I've listended to a few versions since, and this by Ciara Walton is one that I particularly like for the clarity of the melody line:

https://www.youtube.com/watch?v=FYB9QQf90Oo

For any future seekers of enlightenment, here's a link to the studio recording of Liam O'Flynn playing it with Planxty:

https://www.youtube.com/watch?v=EACHnVIl4co