Well it would seem you've already paid the extra money if you've already been allocated seat numbers, I don't know why people bother doing this but that's your call.
Yes, when you check in online, you will be able to print off your boarding card, you then scan this while going through the entrance to security and head straight to the Departure Gates - if it's Aer Lingus regional then you'll actually be going from Terminal 1 not Terminal 2 where most of the other Aer Lingus flights go from.
Depends on the time of day and how busy security is, I got to the airport on Friday at 640pm for a 725pm flight (bus got delayed) and got through security very quickly but I had planned to be in the airport just after 6. I'd normally give myself 90 minutes before the flight.
The flights to Scotland tend to go from the old pier D which is now gates 212-220 so not much of a walk from security.
Be aware my flights were both delayed, by only 10 minutes on the way over but 30 minutes on the way home so if you're getting a bus home from the airport just factor in a bit of a delay.