Two main classes of broadband open to you first of all: landline or wireless. Your choice will largely be dictated by considering whether you want or need a landline - or if you have one in operation at the moment.
wirelessWithin Wireless there are two main operators: Irish Broadband and Digiweb. The former are a heaving pile of shit and should have their license withdrawn because they are consistently unable to provide a guaranteed or reliable service. They're owned by the same crowd who own the West Link toll bridge by the way - they're in it to generate cash, not build a reliable customer base.
Digiweb are far more reliable and reputable. They have a product called Metro which is well worth considering. It starts at €20 p.m and this includes a phone socket on the modem so even though you don't need to connect a phone line or pay line rental to Eircom you do have the ability to make calls - computer doesn't need to be on or anything.
Couple of caveats on metro:
1. It's not available everywhere in Dublin you can find out by ringing them though (www.digiweb.ie)
2. The phone service isn't perfect in my experience. Having said that I'm living on the Northside, miles away from the mast (which is on 3 Rock Mountain) so I'm at the edge of their coverage area.
3. If requires an aerial installation on the roof (small and not intrusive) but this needs to be wired to a socket inside somewhere so it involves drilling - not recommended for rented properties.
LandlinesThere are two main options here for landline based broadband - those reselling Eircom's product (including Eircom themselves) and others offering their own lines.
Eircom provide the national broadband infrastructure through their exchanges so the majority of operators are simply reselling the product that they buy wholesale from Eircom. I haven't been in the market for broadband for a while but the experience up to a couple of months ago would have been:
Eircom - reasonably reliable but not always the most competitive on price. (Speeds: 1MB, 30Euro, 2MB 40Euro, 3MB 54Euro - they also offer a timed one which gives you a limited number of hours online. This goes against the whole point of broadband so avoid it).
Esat BT - usually a much better price, but their billing department is a disaster. Many people have gone a year without paying anything because they can't get their direct debits to run properly. Huge problems for some people so, but their service itself is top notch. (Speed 1MB 20, 2MB 30, 3MB 40 euros)
UTV - I used to be with them and they were the star of the show a couple of years back. They were slow to drop prices to match other operators but I never had any problems with them really. (1MB 21, 2MB 30, 3MB 40 euros - again they do a timed option that should be avoided)
Check all these prices by the way - they're just as I read them.
Your other landline based option is smart telecom. Their offer is very competitive at Eur 35 for speeds of 3MB/s. This also includes line rental to a smart telecom line which is extremely competitive, considering the prices for Eircom, UTV and EsatBT require you to pay line rental as well.
Only catch with Smart is not all exchanges are available - you can check your line (if you already have one) by putting your phone number in here
www.smarttelecom.ie/broadband.php. Also there may be a delay in getting it installed if they have to disconnect an Eircom line first and reconnect you but it's probably well worth it for value.
I wouldn't be overly concerned about the cap. UTV has the smallest cap of the three landline based ones I think and they do enforce it. The cap on Metro is 10GB but in reality they don't enforce it unless you take the piss. I'd say it's a similar case with Smart.
If I was choosing I'd be looking at Smart or Metro if it was available and then maybe EsatBT as my third choice. From experience I had UTV which I had no problems with, I looked into getting Smart but it was only new when I changed and they were talking about 4 month delays so I took Metro.
Finally keep an eye out for special offers. You should be looking for free installation and a couple of months free. Don't pay full whack for installation because there are always offers available from my experience.