I use kemwel.com, but the rate you get from any company will vary from day to day. I assume you are searching the web for rental companies. Spend the time and shop as long and as hard as you can.
Read the webpages or call several agencies every day for several weeks and note the variation in rates.
You will see a trend and then known which one to rent from. I sometimes use toll free numbers to talk with an agent
The size of car depends on the amount of luggage you carry. The webpage will show you the number of passengers and number of bags each car will handle.
Of course you should be comfortable with driving on the left side. That will be your greatest challenge if you have never experienced it. That takes much concentration while you drive.
For Ireland I suggest the smallest car you can fit everything into because of the narrow roads and streets. I usually request a compact or economy size.
Opinion varies about the CDW. Depends on how comfortable you are with the risk. I find driving conditions in Ireland more hazardous than driving on the continent.