Supply of bike parts can be a problem, unless you are looking for a wear and tear part. I was in need of a derailer and had trouble to find a bicycle shop that could provide the one I wanted. In spite of the great nature in Ireland and the good chances for cycling, it is not as popular as in Germany. This might be partly due to the strong winds, the change of the weather as well as bad road conditions on smaller roads. In order to survive these smaller roads it is a must to have your tires under full pressure and the luggage on your bicycle very well balanced.
Low riders are highly recommended.
If you should use a mountain bike it might be worthwile to use one with at least front suspension. Roads in Northern Ireland are a little bit better and since the political situation has become stable, it is no problem to cycle through parts of Northern Ireland. To prepare your bike for the flight, the airlines require that you take off the pedals and turn your steering bar sideways. Also, you have to deflate your tires completely. It is very wise to pack your bike very carefully.