There is a long discussion of the song, and several tunes, in the thread called Celtic Music.

I don't know what to make of the fact that the English version above has 5 verses while the Irish has only 3.

Here's another translation (with 6 verses!) copied from http://www.geocities.com/Nashville/Opry/9441/irishlovepoems.htm
(The site says, "Thanks to Fiona Hyland.")

Roisin, have no sorrow for all that has happened to you
The Friars are out on the brine. They are travelling the sea
Your pardon from the Pope will come, from Rome in the East
And we won't spare the Spanish wine for my Roisin Dubh

Far have we journeyed together, since days gone by.
I've crossed over mountains with her, and sailed the sea
I have cleared the Erne, though in spate, at a single leap
And like music of the strings all about me, my Roisin Dubh

You have driven me mad, fickle girl- may it do you no good!
My soul is in thrall, not just yesterday nor today
You have left me weary and weak in body and mind
O deceive not the one who loves you, my Roisin Dubh

I would walk in the dew beside you, or the bitter desert
In hopes I might have your affection, or part of your love
Fragrant small branch, you have given your word you love me
The choicest flower of Munster, my Roisin Dubh

If I had six horses, I would plough against the hill-
I'd make Roisin Dubh my Gospel in the middle of Mass-
I'd kiss the young girl who would grant me her maidenhead
And do deeds behind the lios with my Roisin Dubh!

The Erne will be strong in flood, the hills be torn
The ocean will be all red waves, the sky all blood,
Every mountain and bog in Ireland will shake
One day, before she shall perish, my Roisin Dubh.