In 1963 my sister taught me to ride on her girl’s frame (no wonder I shave my legs) Schwinn it was blue and it weighted a billion pounds. – Gone, 2nd bike - a Schwinn Colligate (Gold) 5 speed – Traded in, 3rd bike – 1971 Schwinn Continental (Maro
I worked and lived in Ennis, County Clair, Ireland for a year. You can rent bikes in Ireland, just do a web search for rental bikes in Ireland and you will find all you need. Also there are many books about biking in Ireland.
Living there I had a Gary Fisher MTB that I used around town and a LeMond Poprad if I was going any distance. Pick the bike you want for the type of riding your planning to do. So just like America if your touring take a touring bike.
You can take a bike over on the boat with you when you visit the islands.
Roads all over Ireland are narrow and you need to be careful. Follow the books tour ideas and you’ll be fine. Take Rain Gear!!! After the 87th day I stopped counting the number of days in a row that it rained.
Only the major cities have good bike shops. Finding a water bottle was tough if your not in a big city. (There are only about 5 big cities in Ireland.)