I wasn't aware that I needed to check in on line with Aer Lingus, I have a flight booked from Cork in November and the booking confirmation says "confirmed" with Ref No. I thought I just presented this to the check in desk. I cannot see anything about checking in in advance.
Agree with Gipimann, just make sure you are on time. Dublin is not so bad to get through nowadays, it's all much more steamlined. But I guess that can also depend on the flight times.