I've lived in New York City and Boston, but I would never ride a bike in Dublin. First off the taxi drivers alone are agressive drivers and everyone else drives the same. The streets are very narrow and you have as much chance of having someone walk in your path that hit you with their vehicle. I would not recommend riding a bike in Dublin.
Dublin has recently (Sept 09) launched it's shared bicycle scheme. It's modelled on the version in Paris, and unlike those wonderful schemes that use to proliferate in the more liberal parts of Europe with 'take and return' as you please schemes, this is more tighly controlled.
The website is listed below. You have to register first (10 euro) and then use a smart card whenever you pick up a bike from a 'station'. The first 30 minutes is free, so it would be possible to never pay more than the annual fee. I believe that a 'tourist version' will be out soon with a reduced joining fee. Beware keeping the bike over night as charges will begin to ramp up, or not returning it correctly - they have you credit card details remember.
I wish the scheme the greatest of success, but I think it is fair to say that there must be a sweepstake going on somewhere based on how many bicycles of the original fleet will remain in operation by Christmas.
Cambridge in the UK tries a sceme a few years back, and all 300 bikes went withing six hours !
Cycling - this is enjoying a resurgence in popularity since the corporation radically increased the amount of cycle lanes around the city. Unfortunately these lanes are not very well respected by motorists so it is still a dodgy mode of transport around the city centre. Railings are supplied in most city streets for parking and locking bikes but wilful vandalism of these, especially if left overnight, is all too common alas. I live in the city centre myself and I just couldn't be bothered having the responsibility of looking after one!
There is about 200 kms of cycling tracks, paths and bus lanes that cyclists can use, and several place in Dublin where you can rent a bicycle if you don't have your own, as well as the Coca Cola Sponsored ones. Cycling is a great way to see the city, and the Coca Cola Dublinbike scheme is well worth looking into. More information in the link below.
I was very pleased to see a Bike Sharing Service in Dublin. It's necessary to purchase a 3-day card or a Long-Term Hire card to use the facilities, but it is well worth the cost. A 3-day card is currently about €2 and a Long-Term card is €10. There are 44 rental locations around the city center. The first half an hour of rental is always free and a 4 hour rental is only €6.50 so it's a great way to save money on transportation around the city center. Each bike is equipped with a basket for easy transport of any purchases you might make on your journey.