"It's the old way that they used to sing long ago."  -Saili Gallagher of Rann na Feirste
Sean-nos means singing in the old style. Or traditional singing. Or singing a cappella. The term almost defies definition. What everyone can agree on is that it refers to a type of singing that varies widely in style across Ireland. Like most ancient traditions that spring from the various cultures of the world it reflects the heart of it's people.

Sean-nos singing - A Bluffers Guide
by Anthony McCann The Living Tradition magazine, Issue 24 

Sean-nos in Donegal
by Julie Henigan

Amhranaiocht ar an Sean-nos
By Tomás Ó Maoldomhnaigh Treoir, Volume 36 Number 1

About Sean Nos Music
Quoted from Tomás Ó Canainn Traditional Music in Ireland, from the website of Finola ó Siochrú 

Traditional Gaelic song and singing sean-nós
A study of both Scot and Irish, by Craig Cockburn, Silicon Glen.com

Sean Nos
The Wikipedia entry for Sean Nos

Sean Nos and Vocal Music
From a series of articles about Irish and Celtic music by Christina Roden on RootsWorld

Sean Nos - What is it?
From a series of comments (many with links) on The Session
(Note: This is a discussion thread and there will be lots of non-topic banter. Use your discretion!)