I don't think "prestige" is a particularly relevant consideration for medical courses, be that in the UK or Ireland, all of which are accredited by the medical councils and all I think produce fairly standard output. Compared to other disciplines, your personal qualities will count for far more when it comes to looking for a job.
That said, RCSI is well known everywhere, as compared to the universities in Ireland and the UK, its graduate population is larger and more mobile. That means you will meet RSCI alumini everywhere.
Another (minor) point is that some of RSCI's expertise in some specialities may stick to you - brain surgery is one of their feats (the current government has asked Irish schools to specialise more in certain areas, not compete). But then if you want to be a GP at the end of the day, this won't go far.
Just like electricians and plumbers, it is your competency at the end of the day that counts - less important how you got that, compared to say, literature, where class will rub off more. Your degree gives a fairly standard certification of that, wherever you graduate in the British Isles.
Much more important is how you like the teaching atmosphere, the other kids on the block (Stephen's Green is a great block by the way!), Beaumont hospital (you will probably spend a fair amount of time here), the city etc.
Personally I'd prefer to work in St James (TCD) than Beaumont (unless I was a brain surgeon). But then it is a bit like saying I prefer raspberries to strawberries.... at the end of the day, you are full, and both are good!
Go visit !!! Go visit !!! Go visit !!! Go visit !!! Go visit !!!