I have read elsewhere people suggesting the spring or fall, due to there being fewer tourists and consequently fewer tour buses in many of the popular areas like the Ring of Kerry. It's also warmer in the spring or fall than the winter, and summer isn't exactly scorching. In other, less popular areas, you could cycle at the peak of summer and traffic wouldn't be a problem.

Rain is always going to be a factor, so if you don't like the rain you won't be very happy.

I have only cycled in Northern Ireland. If you don't like hills then don't cycle in County Down. Other parts are flatter I hear.

There are sailing clubs everywhere, so you won't have to look far. Best ones seem to be in County Down, oddly enough.

Good luck!