hey. i'm a UCD student who just moved out of merville last month (yay!), but i also spent loads of time at glenomena too. sluequee (ain't she great?) gave some fairly accurate descriptions. merville's your basic 4 single bedroom apt-styled rez, one "large" sitting room with an attached kitchen (comes with oven, stove, toaster and kettle), and 2 bathrooms. so all in all, it's not a bad situation. the walls are thin, the windows have these bizarre vents which lets the cold air seep in (but can be taped up to stop the draft, save on heat and money!).
gleno is 6 single bedrooms with ensuite bathrooms. 3 guys, 3 girls. larger and comfier sitting room, and they get cable tv (but you have to supply the tv). their kitchen is larger and nicer, has a stove and microwave, but no oven. most people i spoke to preferred the oven over the microwave. but it's more expensive and you'll end up paying more in electricity too. although it is quieter and nicer, and it's closer to the laundromat (or as they call it, the laundrette).
the good things about merville is that you'll be near a good number of people from your class, it's convenient in its location (for everything needed when you're not at ET), and it's a great way to meet and mingle with irish and non-med people (which unfortunately doesn't get to happen that often).
there are tonnes of buses that can take you into town and to ET (10/A, 46ABCDE, 145, 746, 84X, there are more, but i forget them). you'll get used to the commute, and it's not that bad.
as for books, definitely wait until you're in dublin. buy books from upper year students. use the books in the library. if you have any of the suggested books already with you in canada, bring 'em up to save money, but get a feel for what you think you'll actually need once you're here.
i hope that helps. once you've got the living situation figured out, you'll find out everything you need to know about UCD when you do the orientation in september. usually the canadians get together after for an informal session to give the new canadian students more useful info.