In the interest of being fair it's best to note that I had issues with the Enterprise office located in West Dublin. Best to not paint them all with the same brush!
Another option is http://www.vanrentals.ie they're based near the Red Cow. I used them once in the past and found them to be very good.
They do weekend specials, where you can pick up the transit on Saturday morning and don't have to drop it back until Monday, for Eur 150. Brand new vans too.