Sean-nós singing is a highly ornamented style of solo, unaccompanied singing defined by Tomás Ó Canainn as:

...a rather complex way of singing in Gaelic, confined mainly to some areas in the west and south of the country. It is unaccompanied and has a highly ornamented melodic line....Not all areas have the same type of ornamentation—one finds a very florid line in Connacht, contrasting with a somewhat less decorated one in the south, and, by comparison, a stark simplicity in the northern songs...

Ó Canainn also asserts that "...no aspect of Irish music can be fully understood without a deep appreciation of sean-nós singing. It is the key which opens every lock".

Alternatively, it is simply "the old, traditional style of singing" and therefore is not always ornamented. It varies very much from one part of the country to another, as according to Hiúdaí Ó Duibheannaigh, who served on the Irish Folklore Commission from 1936 to 1939, "...people now, that word being used these last forty years, think it's a particular style of singing: it's not!"

Sean-nós songs can be relatively simple, though many are long, extremely stylised and melodically complex. A good performance classically involves substantial ornament and rhythmic variations from verse to verse.

Ó Canainn identifies most ornamentation as melismatic ornamentation. This is when a note is replaced or emphasised by a group of adjoining notes, unlike intervallic ornamentation, in which additional notes are used to fill up an interval between two notes.

Decorative elements common in sean-nós singing include:

Highly ornamented singing where the voice is placed near the top of the range
Nasalisation
A second form of nasalisation, used in the south, produces an "m", "n" or "ng" sound at the end of a phrase
Singing one syllable in a word to several notes
Brief pauses initiated by glottal stops, "slides" or glissandi (predominantly when sung by women)
Very long extended phrases
A tendency to draw breath after a conjunction or linking words rather than at the end of a phrase
Ending some songs by speaking the finishing line instead of singing it.

Varying the melody in each verse
An example of the sean-nós singing style, sung by Bridget Fitzgerald, may be heard here.

All these strategies serve an assortment of aesthetic purposes, such as:

Connects the text to the interpretation of the melody
Enhancing a sense of continuity such as by filling the gap between phrases with a nasalised drone
A number of songs are modal, as opposed to major, in melody