The best thing to do is leap into a server and see how it plays.
There's a list of Irish servers in this forum, here: http://www.boards.ie/vbulletin/showt...p?t=2056223274
That'll give you the basic information about the servers, most important of which is the IP address. When you click on the multiplayer option in Minecraft, you'll be asked to enter this. And once you do the server will load up. But before you do that, you ahve to get permission to access the server, which is what "whitelisting" means. In the Minecraft forum, you'll see threads for the various servers. Just post in them asking to be whitelisted and give your Minecraft username. Once a server admin sees your request, they're fairly quick to add you and tell you that they have. So once that's done, you can access the server.
As said above, each server can use a variety of mods to change the way the game plays. Now, I'm no expert on what mods are used and what they do, so if you've questions about a specific mod that's used in a server, then ask in that servers thread. People are happy to help. You can also get help in game by pressing the "T" key by default and typing /help. This will list some of the commands available on a server. If it tells you that it's on, say page 1 of 8, then just type /Help 2 or /Help 3 to see the other pages. Some of the commands won't work as they are only usuable by the admins, but just experiment if you want. I don't think there's any real way to cause any trouble with the commands.
The Town mods do various things depending on how they're set up and which ones are used. You can set it up so that a town will have benefits like you heal automatically while you're in there, but these benefits stop once you leave. And some servers are set up so that the various towns can fight each other for dominance and to expand their territory.
The money is used for buying various things in the game. On the Boards.ie server you gain money from killing monsters and some players have shops in the main town where you can buy anything you can't find in the world. Personally, I've not bought anything as I've never had a problem finding stuff, but if you're in a hurry and struggling to find something, it can be handy.
If you have mods installed for the singleplayer version of the game, you may have to disable them as some servers pick it up as cheating, even if you're not using them.
Each server will also have it's rules. The most important one, which is pretty much universal is: Don't be a dick! You'll find many users who join a server just to cause trouble, or "grief". They get found out pretty quickly and are kicked off the server. Also be respectful of what people build. If you destroy something of someones then try to rebuild it or if you can't let them know what happened by leaving a sign there, or by posting in the servers thread. Accidents happen, so there's no problem with being honest. And if you're unsure as to whether or not you can build something, just ask in-game or on the thread, and people will help.
And I may as well mention the updates. I don't think there's going to be too many more of them, but Minecraft is in Beta stage at the moment and isn't a fully working release. So Notch (the guy who created the game) brings out a new version of the game every now and again, and when you start Minecraft it'll ask if you want to update. The thing is, the servers don't update as soon as the game does.
The servers mostly use a program called Bukkit (or I think that's what it's called) which allows them to install various mods easily. When the game is updated, then the guys who created Bukkit have to update that to work with the updated game, the guys who created the mods have to update their mods to work with Bukkit and the game and then the server admins have to install the newly updated Buckit and mods and then you'll be able to use it again.
The short version of that is, don't update your game until you're told the server is updated.
Oh, and one final thing. As I said, the game is in Beta, so it's not fully stable, particularly in multiplayer. So you might find yourself kicked off servers from tmie to time, or that you can't log in. Just check in here to see if a server is down, or if anyone else is having trouble.