James,

It's a taste thing, but I really prefer the singing to be totally unaccompanied as that's the sean nós style. One of the things about sean nós is that the rhythm is quite free, and in particular the pauses for breath at the ends of the lines tend to be varied according to the mood. So mys suggestion would be to alternate between singing and instrumental.

If you're doing your own mandolin accompaniment, you can obviously judge the tempo and the pauses. But if, as your post above indicates, someone else is doing the accompaniment, this can tend to box you into a rigid tempo which detracts from the character of the singing. So if you do go for accompanied singing I would suggest that you make it clear to your accompanist that s/he should follow your lead rather than impose the rhythm on you with the mandolin.

And without being excessively politically correct, could I suggest that if we do come up with an English translation that fits the tune, we should substitute "God" for "Christ" to widen its appeal?