"Sean nós" simply means "old way"; it's similar to "old-timey" and "traditional" in English. But I don't know if one can describe a song as being "sean nós"; rather "sean nós" describes a style of singing (btw, the term is also used to describe dancing). And besides being recently composed, this song is usually not sung in a sean-nós style and it lends itself well to accompaniament.

The author is Tomás Mac Eoin of an Ceathrú Rua, Conamara. He put his words to the well-known Mingulay Boat Song tune. A Scottish singer called Art Cormack heard the sung being sung by Éamonn Ó Faogáin in Belfast and he brought it back to Scotland. I believe singer Christine Primrose, a native speaker, helped Art with the translation to Scottish Gaelic. They recorded it with the group Mac-Talla on the album "... Mairidh Gaol is Ceol" (Temple Records). This song is also recorded by Donald Angus Mathison ("Eilean Mo Chridhe", Smith/Mearns Recordings, Perth, Scotland), and I think Iain MacKay sings it as well. I have often heard "A Chailin Àluinn" sung at cèilidhean in Gaelic-speaking areas of Scotland; it has become very popular there.

A CHAILIN ÀLUINN (translated from An Cailín Alainn)

A chailin àluinn dha'n tug mi'n gràdh-sa
'S i fhèin as àille na blàths ' nan ròs.
Gun i bhith làimh rium, 's ann tursach tha mi;
A chailin àluinn 's tu fàth mo bhròin.

An am dhomh dùsgadh is mi nam aonar
'S e sin an uair as moth' mo bhròn.
Bidh mo smuaintean air a' chailin uasail
A dh'imich bhuam-sa 's a rinn mo leòn.

A chailin àluinn gun tug mi gràdh dhut,
Thig na mo chòmhnaidh mo luaidh 's mo stòr
'S abair rium-sa gur tu mo ghràdh geal
'S bidh mise àghmhor gun aobhar cràidh.

Nach tig thu leam-sa, a chailin àluinn,
Gu sìorruidh bràth cha bhith ort bròn;
Sheinninn ceòl dhut mar cheòl na clàrsaich
'S mar ghuth na smeòraich an driùchd an fheòir.

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The following translation is (slightly adapted) from the Mac-Talla album notes:

O beautiful girl to whom I gave my love,
She alone has more beauty than the fair roses;
Without her beside me, I am sorrowful.
O beautiful girl, you are the cause of my sadness.

On waking, when I am alone,
That is the time I am most sad.
My thoughts are with the noble girl
Who went from me and wounded me.

Beautiful girl, I gave my heart to you;
Come to me, my darling and my treasure.
Tell me that you are my fair love
And I'll be joyful without reason for torment.

Won't you come with me, beautiful girl,
And forever more you'll know no sorrow.
I would sing to you like the music of the clarsach [a small harp],
And like the voice of the song-thrush in the dewy grass.

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In a modern dictionary, "Brith nam Facal", àluinn is spelled àlainn and sìorruidh is also spelled with an "a" instead of a "u", more like the Irish spelling. By the way, I notice that the Scottish translation retains some words - such as cailín - which are known in Scottish Gaelic but not so commonly used as they are in Ireland.