my opinion would be to go with the credit union. a lot of people get into trouble with car finance companies. their interest is also very high and you cannot be flexible with paying a little extra off the loan when you can afford it. also, each of these companies are registered with the irish credit bureau so if you miss one payment it is recorded.
Quite a few people get into trouble with CUs as well. THey will pursue you for outstanding debts. One of Eddie Hobbs 'Show Me The Money' programmes a couple of years back featured a young lady who's CU had sought a commital order to send her to jail for unpaid debts.