I wish to have a short (co) Irish verse on a grave headsone for parents and brother. I have been advised of the following which I would like a second opinion on before going further.

Go dtuga Dia suaimhneas síoraí dóibh/dá n-anam

I'm a little unsure if it shpuld be : Go dtuga Dia suaimhneas síoraí dóibh
or
Go dtuga Dia suaimhneas dá n-anam
I would be very grateful of any thoughts on the above wording.

That’s your choice:
Go dtuga Dia suaimhneas síoraí dóibh - May God grant them eternal peace
Go dtuga Dia suaimhneas síoraí dá n-anam - May God grant their souls eternal peace

Go dtuga Dia suaimhneas síoraí dóibh - May God grant eternal rest to them


Go dtuga Dia suaimhneas síoraí dá n-anam - May God grant eternal rest to their souls.

The previous suggestion looked like a google translation result.