Translation would be:
[i]Your help* now O Gods, O ancestors, O spirits,
if it is your wish;
as we offer this sacrifice
for the sake of life**[/i]
* Eisidh is "to beseech, pray, beg, implore, invoke help of ". So in essence the prayer is invoking the help of the Gods, ancestors, spirits. You could translate it as "We beseech you" "we ask you" etc. if you wished. It is a call upon the Gods for help in granting the subject request of the prayer.
[Just a note on "sacrifice" as it's a potent word - it does not have to mean an 'animal sacrifice'...it can refer to any offerings made in accompaniment to a prayer petition - food, wine, milk, candles, objects of value, etc.]
** "beatha" is "life", "sustenance" "livelihood", etc. so you can translate it either way accordingly. eg. if it was a prayer for a good harvest, I'd probably translate as "sustenance" rather than "life". One could even translate as for health in the sense of beatha as life, vibrancy etc.