Just a few quick tips on driving.
First, a good car rental company I discovered on a package trip a few years back is Dan Dooley Car Rental. For some reason, they aren't well known outside of Ireland but I have found them the cheapest and with a bigger fleet in more locations than the usual majors. On subsequent trips, I've rented from them again and been very happy with them.
Second, if you're not used to driving on the left (See - I didn't say wrong) side, I would recommend paying the extra for an automatic. Shifting with the left hand while learning to think left sided is one task too many, especially in the larger towns.
Thirdly, roundabouts aren't as tough as they look. It takes planning and careful merging to avoid either being stuck circling for hours or upsetting those that know what they're doing. If traffic is heavy - the outside lane is for turning onto the next street (90 degree) or straight (180). The inside lanes are for going either straight thru(180) or the 3rd turnoff (270). On light traffic, things are looser.
Finally, avoid driving in the larger cities (esp. Dublin and Limerick) as much as possible. The roads aren't built for the traffic, finding parking is a pain, and it really cuts into your drinking! Drive to wherever you're staying and use public transport.
You need a car to explore County Clare and County Limerick. And although major roads connecting major towns are getting pretty good, the rural roads of Clare are still subject to Irish Traffic Jams. These cows shared the evening's twilight with us.
As for driving around Limerick City, well my Dad wont do it unless it's strictly and unavoidably necessary. :)
Downtown Limerick comprises mainly of one-way streets, so if you're new to Limerick, best to get a traffic map or drive slowly and observe what the best lane for you should be.
My main advise here is try not to take your car around the Market areas, as it's heavily pedestrianised
At the moment, the Limerick city corporation are building a bypass around the city which should help the congestion in the outer suburbs. Eventually the Nenagh - Limerick stretch of the N7 route (the Limerick-Dublin road) will be improved and possibly upgraded to a motorway.