Plan on rain everyday

that way, a day without rain will seem like a bonus

I've never ridden in Ireland but, I've ridden in England & Wales and the weather can be "unsettled" as the Brits like to say meaning, you can go from sunshine to gully-washer and back to sunshine in just a few hours.
Just plan on rain everyday, that way you won't be disappointed

That said, when the sun is shining, the UK is a great place to ride a motorcycle.
Many of the backroads will probably be single-lane so, that will keep your speeds down but, they're preferable to the main arteries.
Edit to add: R Doug posted pictures of his trip to Ireland in this thread:
http://www.sport-touring.net/forums/index.php/topic,59861.0.html