I had similar concerns, but it's not as tough to get used to as I had feared.
I rented a car in Ireland, and drove around the entire island for two weeks. After the initial drive from the airport to the hotel, which was somewhat stressful as it was an urban area, I found I quickly adapted to the change, and didn't have to remind myself which lane to be in.
My only issues were the roads. If you get off the main roads, many of the roads are barely wide enough for 2 cars. The Irish seem to drive like maniacs (very fast), and with the lush growth on the side of the road (with no shoulders), the tendency is to move in a few feet from the edge, which doesn't work when someone is coming in the other direction at 60+ mph.
Interestingly, when I returned to the USA, after picking up my car at JFK, I started to go in the wrong lane when I exited the parking lot. A loud horn jarred me back to driving on the right.