Backpacks

I can't speak to aerlingus, but know that when you are touring, a lot of places /museums /sites will make you check a back pack but a messenger style bag is ok. There is a thread rt now under packing on "manly shoulder bags "that discusses this in depth

Some airlines have specific size limits for your "personal item" where carry-on baggage includes one carry-on and one "personal" bag. As a rule of thumb, the personal item should be able to fit under the seat. Under those conditions, a small backpack works but a full size will not be a "personal" item. Check your air carrier's cabin baggage rules.

We just returned from Ireland on Aer Lingus. I always carry a small backpack instead of a purse, and it has yet to pose a problem on any airline thus far, and it did not on Aer Lingus. On our return my husband had both a backpack and a bag from his duty-free shopping. The shopping bag went into the overhead bin and the backpack beneath the seat in front of him. Neither posed an issue.

My wife and I always use backpacks as carry-on bags. We generally fly U.S. carriers or British Airways and it hasn't been an issue.

My wife and I both have the Osprey Farpoint 55 travel backpacks. Basically, these consist of a 45L main pack and a 10L detachable daypack. On our 2 week trip last summer, one of us used our daypack for daily activities and held all we needed for whatever we were doing in Greece, Austria, Germany last year. Planning to do the same in Ireland next month. They counted as carry-on luggage on Delta, Austrian, and Aegean Airlines as we were well under the weight limits.

I'd recommend it over rolling or framed luggage, as the ergonomics were superior and traveling with less is truly liberating.

I always use a Daypack for my carry on, and so far there have been no problems with that. You would of course have to be sure that the pack fit the size & weight criteria of the airline(s) you're using.