Mike84,
You should have little problem returning to Canada after doing a FP residency in the US. The FP residency program in the US is one year longer than in Canada so recognition of training length and quality will not be an issue. You will just have to write your Canadian exams to achieve licensure.
There are two ways that this can be done. You can write your Canadian exams while practicing in the U.S. then return to Canada when licensed. Or you can obtain temporary licensure in Newfoundland or Saskatchewan and immediately practice prior to writing your exams. You then have several years to pass your exams and obtain permanent licensure. At this point, you could remain in Newfoundland/Saskatchewan or migrate to another province.
The most common route for Canadians studying in Australia/Ireland to return to Canada is via the U.S. This is a very realistic and viable route as the U.S. has several thousand residency positions that are open to foreign students (unlike in Canada) since the U.S. views these foreign residents as a cost-effective way to supplement their health care staffing. This is a very different situation to Canada/CARMS where there are 100-200 surplus residency positions open for the second round.
The other option for returning to Canada is by doing a residency in Australia. Australian credentials are recognized and approved by the Royal College of Physicians and Surgeons of Canada (see
http://rcpsc.medical.org/residency/certification/img_e.php). Once again, all that would be required is to write your Canadian exams.
In summary, going to Australia/Ireland is a very viable route to enter medicine and is not at all a dead end. One must just be realistic and patient about the timeline to return to Canada and fully understand that you will likely not be able to achieve a residency in a competitive specialty. Personally, I view an extra few years in the US or Australia as part of the adventure and not wasted years by any means.
If on the other hand, you have a very pressing need to limit your time abroad to four years only - there is significant risk that you may not win one of the spots offered in the second round of CARMS. I do believe the odds are better than the 13-14% quoted in CARMS statistics as I would also speculate that recent grads from countries where the medical education is perceived as equivalent to Canada are preferred. However, in my mind, the risk is still material that one would not get a Canadian residency and would have to go the US or Australian route. If being back in Canada ASAP post-grad is an urgent need, then it may be worth an extra round at the Canadian Medical School Admissions game.
Hope this helps and good luck!