Eddie Hobbs thinks he has an answer for everything. Buying a second hand car in UK and importing it to Ireland, will almost always result in a cost saving v the price of same second hand car in ireland. I only ever buy my own/wifes/family's second hand cars in UK. Of course, you need to make sure you aren't being ripped off on the price in the UK, but a quick trip to
www.autotrader.co.uk will give you the price range in which you should be trading.
In my case, I have a sterling account which I have maintained from my time in the UK, so I can just transfer the money electronically in advance, getting a best negotiated rate from my bank. I would avoid using a credit card at all costs, you will get clipped for about 4% off the interbank rate. With current cheap flights, you can afford to travel twice, leave a deposit, and return with draft if necessary. If you want the best deal however, go as a cash buyer if you are comfortable carrying that amount of cash. Best negotiating position, you can discount all sorts of things as you aren't interested in their warrantees etc, which all dealers will have built into the price.
You can travel to one of the big second hand dealers in London who have thousands of cars, you're sure to find what you want, and you can have the draft made out in advance, with maybe a couple of grand in cash to make up the negotiating elements.