Entering canada without a return flight (visitor). Leaving and coming back

Is there a particular reason for not holding a return ticket for this next trip? Is the length of the trip totally undecided? Have you always held a return ticket on previous trips?

Without wishing to directly contribute to your illegal activities in Canada, is there any impediment to your purchasing a return ticket where you can change the date for little or no cost? Or perhaps even cancel the flight for a full refund?

What stumbling blocks if any can I face here?

Repeated stays or any sign that you are working/generating an income or building a relationship might suggest that you are not genuinely visiting Canada and might be tempted to stay there illegally or otherwise squirt the rules. Not being able to demonstrate clear ties to your country of origin (you apparently have no job and no partner in Ireland since both are in Canada…) compound the problem. You could therefore be denied entry and “invited” to apply for a proper immigrant visa/permit as applicable (with no guarantees that this would work either).

Should I mention I am there primarily to play/gamble in poker?

Playing poker (and seeing your lover) certainly sound like material facts. In this context, claiming to be “visiting friends” (or worse, “doing tourism” as other people sometimes do) is deceptive, especially when you are really going to Canada primarily to play poker, per your own admission.

In principle, you should explain what you are really doing in Canada or at least be comfortable doing so, even if you don't volunteer any information without being asked. The fact that you have not been entirely forthcoming in the past suggests that you are (rightly) concerned it could be a problem.

Also will I have some problems getting into Canada without a return flight?

I don't know whether holding a return ticket is a strict requirement in Canada but not having one can often be a problem in other places. It's particularly important for you because you want to avoid extra scrutiny, which could raise questions about your past stays and bring up all the issues I mentioned before. The ticket is not even your most pressing problem, playing fast and loose with it is a very bad idea in your situation.