Assuming that your flight is with Ryanair, the normal minimum ckeck-in time won't apply (because you'll have checked in beforehand) so, theoretically, you're only constrained by the rule that you should be at the gate 30 minutes before departure. In practice you can ignore this; as long as you're there 15 minutes before departure, you'll be able to board. (I've been allowed to board a Ryanair flight 9 minutes before the take-off time but that was definitely 'cuttig it fine'!).
So, you arrive at 0820 and you need to be at the gate by 0920. It takes 5 minutes to walk from the coach stop to the end of security queues, probably 15 minutes maximum to get through security. Add on a generous allowance of 15 minutes to get to the gate. That means that you've got 25 spare minutes built into your day. As long as the coach is on time, you shouldn't have any problems.
Chris