Maybe some local input might help. Previous posts seem to paint a stero-type picture of Ireland.
Firstly, all major car rental companies are represented in Ireland and Dublin in particular. At Dublin airport you have Avis, Hertz, Alamo, National, or their equiv franchise. See:
http://www.dub.aero/AR_Dublin/live/Lv_pres_GenTemplate.asp?strPage_Name=DN_CarHire
All companies carry a full range of cars from economy to luxury.
We have a good national road system, and one of the most densely covered road networks of any country. (It does require some maintenance to keep all roads up to spec).
Petrol is among the cheapest in Europe, about 25% cheaper than UK. About 1 to 1.05 Euro a litre.
Dublin is a small city, still retains a lot of its older buildings, narrow streets, and heritage. Not designed for modern car, or volume of traffic. Loads of one ways streets, it can be a bit of a maze. Anyway you dont visit/tour Dublin in a car, use your feet. Most of the key city attractions are in a 1 mile square area.
Cant remember the last time I saw sheep on the roads, again, if you travel up country lanes, always expect to meet some country lane inhabitants.
You dont have to go for a small car, unless you plan to drive up a lot of country lanes. If you are tripping then I am sure you will stick to either the Motorway, National Primary or Secondary roads.
Smaller roads are winding, so your can enjoy more of a driving experience, keep to speed down, and enjoy the scenery.
The south is a lovely place to visit, so if you need any pointers, drop me a line.
BTW I would not bring my Pcar on a tour of Ireland, not after my recent suspension woes.
Failte romhat!