If you rent an Automatic, you'll be given an Automatic. If you don't rent an Automatic, you will most likely be given a manual.
In countries like Ireland (presuming you mean Killarney, Ireland?), the majority of rental cars will be manual, and renting an automatic will almost certainly be more expensive. On a rental I had in the UK a few weeks ago, the prices for an automatic ranged from 2 times to 5 times the price of a manual, and at least one company was "sold out" of all automatics whilst still renting manuals.
As with any rental cars, there's always the possibility that they will not have what you've booked when you arrive. This is normally caused by people not returning vehicles on time, leaving the location short of a specific type of car. This is no difference regardless of whether you book an auto or a manual, with the exception that as companies in the UK normally have less autos available there is potentially a greater chance of this happening when you book an auto. If this does happen, you should ask the company to make you a booking at a different rental company and cover the additional costs (if any).