She/he doesn't need MCAT if she's doing undergraduate entry. MCAT is only required for graduate-entry (as in coming in the 2nd year of a 6 year course) . I'm not canadian...but i've actually never known any canadian high school leavers to apply for the 6 year program...all the canadians I know either did graduate entry, or they got part-way through their college years and decided not to continue doing undergrad studies at their universities, but to enter irish med instead as an undergrad student. (So they got like 2 years through college, then dropped out and switched to studying medicine in Ireland).
The Atlantic Bridge application specifically asks for research experiences, so I am assuming having research as an extracurricular helps a bit. They also require fairly decent grades...they don't have a specific cut off, but it seems most people applying successfully have at least around a 3.4 or 3.5 GPA.
To my knowledge, the irish schools specifically want north american students to apply through Atlantic Bridge. My impression is that Atlantic Bridge students get allocated a specific quota (as in, there's a set limit to how many americans/canadians they accept, and that number is drawn from AB applicants). I have heard of others going outside AB, but don't really know much about that. on RCSI's website, I think I saw they give the link to AB in order to apply.
I'm not really sure about the advantages of AB, except they are very quick to answer queries...and it can be nice to have a middle man tell you when mail is coming. Correspondence with the schools themselves can actually be a little spotty.
