I think you may mean Dublin Coach www.dublincoach.ie
I am not familiar with this company. But, it might be like booking a seat on those high speed trains in Italy. If you miss the particular bus your pre-booked ticket might not be valid for the next bus. Also the bus might be delayed arriving at the departure bus stop. If you need to link up with your tour at a specific time it might be worth going 30 minutes earlier to the stop to allow for delays. You could read the company terms and conditions. Having a pre-booked ticket may not guarantee being able to get on the bus.
Also, a backup plan would be www.irishrail.ie
for trains from Dublin to Ennis. Sometimes there is a bus transfer to the train station, Dublin Heuston station. Download the timetable for Dublin to Ennis close to time of travel. Check for any change of train. Sometimes there is also a bus transfer from Limerick Junction to Ennis. Check for timetable changes and service updates.