Doubt Donnacha O'Callaghan would take kindly to being told his name is feminine! But I guess if you live in the UK, I suppose it might be a tiny bit confusing since '-a' names are so huge for girls just at the moment. Personally, I think it's a very strong name and that if you stick with it, people will soon get used to it.
I can't believe people have been rude enough to criticise new baby's name...I hate that. I had a lot of mean comments about DS's unusual but very pronounceable and easy to spell Irish name when he was born (prematurely and under dramatic circumstances), and they still rankle when I think about them. Not because I'm still worried about his name - he's 6 now and totally owns it - but because I still think how damn insensitive the people who made them were.
I like Iarla too - it makes me think of the flight of the Earls and the Wild Geese and all that - though not quite as much as Donnacha, which makes me think of strong limbed, swarthy rugby playing types. The Iarlaith spelling I would probably avoid outside of Ireland and I would worry that the pronunciation of both Iarla and Iarlaith might be a challenge.
If you're really set on looking for alternative Irish names, how about Ronan or Rory or Senan or Felim or Lachlan? Or would Donagh (pronounced Done-ah) pose the same problems? If it was me, I'd stick with Donnacha though.
Congratulations on your son, by the way. Hope the breastfeeding gets easier! winces in anticipation of doing that again