Despite their faults Ryanair are about as punctual and reliable as you can get. In fact their flights often arrive early (I suspect that this is because they advertise flights as lasting longer than they actually do in order to massage their punctuality statistics but the result is the same).
As long as you're traveling without checked baggage you should be okay. Having said that it might be a good idea to book a cheap flight from London to Dublin a day or two later in case anything goes wrong- at least then you'll be able to get back across the Irish Sea without paying an extortionate amount of money for a last minute flight.