I looked into both quite extensively but let me preface this by saying im still pre med and not an authority on the matter.
I would think that Ireland would be the easier route to come back to canada. Really, only for 3 reasons:
1) Alot of canadian students attend irish medical schools because of the intensely competitive nature of canadian medical schools. This being the case, Canada knows what they are getting as they already have alot of them comming back (i know 3 myself though all FP's).
2) For some reason the UK/EU schools are seen as more prestigious than AU schools. In reality there is no truth to that. However, Irish schools such as Trinity, UCD and UCC have a known reputation in Canada. I had even heard of Trinity and UCD before i ever considered medical school.
3) There is a disconnect in the school years in North america and AU. This could be problematic during your match times (you might end up with a bunch of time waiting for the match to start again) as well exam writing times. Not something that couldnt be overcome but something to consider.
Anyway, this is by no means the bible. I would suggest you apply to both and then consider based on where you get in, finiancial issues and personal preferance.