Here's a suggestion. If you know any golfers who have taken a trip to Ireland, see if they've had any experience with coaches/drivers. From researching similar issues for golf trips to Scotland, I've learned that the economies of scale don't start to kick in until you've got a group larger than 12.
UKtravelbear has rightly pointed out one of the hidden costs of such a method of transportation--if you stick with one driver throughout your travels, you have to factor in his lodging and food costs as well as the costs for your travelers and that will either be a separate line item or built in to the quote you receive. Here's another: if your group is 12-15 but the companies you contact can only offer a 19-seater coach, you're probably going to pay the same price for the coach as if your group had 19 passengers.