This is in the book - 'Fairy Legends and traditions of the South of Ireland' compiled by Thomas Crofton Croker. At the end of the story 'The Wonderful Tune' there are four lines of verse of which he says : 'This is almost a literal translation of a Rann in the well-known song of Deardra.'

Beautiful shore, with thy spreading strand,
Thy crystal water, and diamond sand;
Never would I have parted from thee
But for the sake of my fair ladie.

Does anyone know the original version in Irish ?

Great question, franc!
I've always wondered about that one myself.
I sure hope that there's somebody who can answer your question. *crosses fingers*

There's a version of this from the South of Ireland? I only know the Scottish-Gaelic version:

Ionmhuin Inis Droighin de
Agus an uisge gainmhe
Go nach ttiocfainn aisde alé
Ach muna ttaguinn le Naoise.

this isn't Scottish Gaelic, it's a kind of Classical Irish or Early Modern Irish (btw the Scottish writers would write in Irish until the middle of the 18th century, if I remember well)

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Just how old are you, Lug?

Rugadh ins an 16ú haois mé

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne