In comparing the flight times from Bristol to Dublin between Ryanair and Aer Lingus, it seems that Ryanair can make the trip in 55 minutes, a full 20 minutes less than Aer Lingus.
Actually Ryanair have more padding in their timetable with regards to journey times. Aer lingus operate the route through Aer Aran and their ATR aircraft. Ryanair use the 737 jet which is about 200mph faster at cruising speed.
In comparing the flight times from Bristol to Dublin between Ryanair and Aer Lingus, it seems that Ryanair can make the trip in 55 minutes, a full 20 minutes less than Aer Lingus.
It's probably down to the aircraft used as mentioned but a difference of 20 minutes over an hour long flight seems a lot. Given the time needed to reach cruising speed and for landing, I wouldn't have thought the opportunity would be there to make up a 20 minute advantage.
As mentioned, the route is very often used for business purposes where time is more of the essence so it would seem that Ryanair hold a significant advantage. It's only relatively recently that AL have operated the route with such regularity so they're trying to break into a route on which Ryanair has already a significant foothold. It seems strange that they would attempt to do so with what is a diminished service by comparison.