I was looking to buy a Rick Steeves convertible backpack - http://travelstore.ricksteves.com/catalog/index.cfm?fuseaction=product&theParentId=8&id=139 but the dimensions would exceed Aer Lingus carry-on limits - is this correct? http://www.aerlingus.com/help/help/baggageinformation/
Aer Lingus: 22" x 16" x 8" Rick Steves bag: 21" x 14" x 9" If you don't stuff the Rick Steves bag to overflowing, it certainly should satisfy the 8" Aer Lingus depth requirement. That's the beauty of a soft-sided bag as opposed to a wheeled bag with a hard shell.
Many airlines post dimensions and then ignore them if your bag isn't outrageous. I can't speak for Aer Lingus but British Airways and BMI don't really care but Ryan Air (who are currently trying to buy Aer Lingus I believe) make their profit enforcing super strict luggage size restrictions and then charging you to check at the gate. Maybe see if you can get a feeling for how strict they are from other travelers.
I made my Rick Steves bag fit n Ryan by wearing both my jacket and fleece so they weren't in my bag. Plan on how to slim your bag - maybe take a side bag like Rick Steve's shoulder bag - and take some things out if it's too thick.
We flew Aer Lingus in July 2010 from JFk to Shannon and return. My husband and my son both had the Rick Steves convertible bag as carry ons. We had no issue at all. We were careful not to overstuff and did not open the extensioin zipper. We loved flying Aer Lingus! Excellent service! We are leaving for our month in England and France soon and all of us have the Rick Steves Convertible Backpack now. The bag weighs next to nothing so more room/weight for shoes/clothes! : )