I haven't done what you did; I emigrated to Australia 8y ago and am still here, and unlikely to return because DH is Aussie.
But I go back to the UK every year to see family and every time I'm there I get heartsick when I have to leave again. But there are things that are better here too, so it's a tough one now, and worse as the boys are getting older and have their own friends and life here. DH would never leave Australia for the UK again though, certainly not while his mother's still alive, so I don't think I'd get the option - but I do wonder sometimes how I'd fit back into things over there.
I think you need to sit down and write a pros and cons list for both Ireland and Australia, and decide what you want to do before your children hit school age, as the longer you leave it the harder it will be for them to leave friends etc.
Also, if you go somewhere different in Australia, having pre-schoolers is a great way to meet new people, as I'm sure you know!
DH has been offered jobs in different parts of the Antipodes but I'm wrecking his career (not really) by refusing to move again, as I'm just about settled where we live, have friends and a support network, and I'm not about to give all that up again!
Would you have jobs to come back to here? I realise you moved back for the family support, but if you came back to Aus, would you have people you know to come back to?