A good rule of thumb I employ whenever I interview is to realise that I'm interviewing them just as much as they're interviewing me. Therefore, ask pertinent questions, interact with others in your group as chances are these people might make up your crew one day.
And, as always, of course you're going to get frustrated by not seemingly getting to the next stage but most crew have the same experiences. Not many of us got the job on the first attempt.
As an aside, I find your comment about not being the ideal 'physical specimen' slightly wide of the mark. It takes a lot more than an attractive person to be crew...an attractive personality wins hands down every time.
Also, you say you're being greedy for longhaul...I suspect your interviewers pick up on that and see you for wanting the GLAM of the job rather than the Dublin turnarounds. If you really want to fly it shouldn't matter where you go for your doing the job for what it is, not for the perks. The perks can come later once you've more experience under your belt.