This thread's been around since May '17 but just spotted it now. I'm Irish and live in Ireland. I also regularly cycle tour in mainland Europe.
1. I'd bring it as checked baggage.
2. Best time is June or September. Not as busy as July & August but still hopefully decent weather.
3. There are official websites for Northern Ireland and the Republic of Ireland that cater for cycle tourists. Try Google.
4. I'n presuming you're an American citizen so if you want to spend a year in Europe you should first check out how long you will be allowed stay. Europeans visiting the U.S. are restricted to how long they can stay and the same goes for U.S. citizens visiting Europe.