Gaming groups tend to be based in the houses of the group organiser, in most cases, that's the GM. The size of the house tends to determine the max. size of the group.
Joining a group is not the easiest thing. It usually involves getting to know one member of the group and eventally getting a foot in the door when a vacancy arrives. Or it can be as simple as starting a conversion with a stranger on a train when you notice him reading RPG books (true story - the guy in question is GM of one of my groups).
As for me, I run one group and play in another. That works out at 2 Saturdays a month on average. Getting 4 to 6 people to commit to a Saturday afternoon session on a regular basis is difficult. Location is a major factor. I live in Deans Grange (southside), the GM of my other group lives in Skerries (closer to Belfast than Dublin....)
We both have players that travel distances to be at our games. One guy plays in both our games and runs a group of his own. Several of his group also play in my mine. And my GM runs a 2nd group one Friday night a month.
So you can see the logistical nightmare of getting sessions organised. Throw in obstacles like family events or sports and you get further complicated.
As things stand my 'table' is full right now but that's not to say vacancies couldn't occur suddenly and unexpectedly. I've been lucky with this incarnation of the group. It's probably the third such over the last decade or more. It began with three players with a fourth promised to join asap. As that fourth joined, one player left. But we then recruited a player from a previous version of the group. Then we got a fifth and just as we got our sixth, the original two left. Then a player I'd been trying to recruit for years finally joined, We're back up to 5. We have been at 6 for a few months now having gained two more and lost one (who incidentally was in all three groups).
It's a see-saw world being a GM and keeping the lines of communication open. I suppose the nain thing I'm trying to say to you is that it might not be easy finding a group. You have to put yourself out there. Let people know
Working or college
Where you live
Have you transport
How far are you willing to travel
What are your Saturdays like Regularly free or worse
What level of experience with RPGs do you have
Put all that info out there on the forum here and you never know what GM might see it and have seat at a table. In some ways it is like trying to find a seat at a poker school.
The other option is to get out to the conventions, the big two are over for the season, Gaelcon and Warpcon, but there is still the oldest, Leprecon, is due up in a few weeks and luckily for you is a local one, Trinity College. Get yourself in there, It will be intimidating and look like a closed members shop but that's not really the case. It's more that they are in their environment and caught up in the whole thing. But if there's one thing gamers loves to do is talk about games.
Talk to someone and they'll glady give you a demo of a game. Be careful not to interrupt the tournament players though. They're a bit more intense.
Well best of luck.