Just to follow-up, should others have similar queries in the future...

We ended up going with Irish Day Tours (http://www.irishdaytours.ie/) aka Extreme Ireland (http://www.extremeireland.ie/).

Booking online was pretty easy. Slight discount for booking multiple tours, and they do add a 3% fee for booking online (presumably to cover the credit card processing fees).

As with most tour operators, guides varied a good bit in personality and style, but all of our drivers/guides were knowledgeable, kind, and resourceful. For one of our tours, our coach actually broke down at one of the stops, which delayed us for 2 hours. Our driver did his best to ensure that we didn't miss any major stops on our tour and kept our spirits high throughout the day. The tour company offered a 50% discount on a future tour as compensation, which worked for us as we already had a booking for the next day.

And thank you, lwildernorva, for your suggestions! We did end up doing a Belfast/Giant's Causeway/Carrick-A-Rede tour, which was great.