Aer Lingus Carry on Baggage

I flew Aer Lingus to France a couple of times, and I think our carry-ons might have been in the same situation as yours. I'm sure that we were able to carry them on, and I don't remember the measurements being checked; they may have been weighed, though. But there's no way of telling in advance what will happen.
If you're concerned, you might want to check your bag right at the start. There's probably less of a chance of it going astray then if it's gate checked at the last minute. That's the one time when my bag went missing, and I didn't get it for several days.

If the flight is busy, it’s more likely to be checked than if it’s only half full. They have restrictions for a reason and if the bag is too big, there is a risk that it will have to go into the hold. It’s usually cheaper to pay for a checked bag in advance than at the gate. You need to ensure that you also comply with their weight limits. I have seen bags weighed and measured.

Ditto: If it is full flight (and also if inconsiderate passengers get on before you take up all the bin space by shoving their winter coats where your bags should fit) then you might have it checked at the gate. All of the airlines have reviews on social media about lost luggage, rarely do passengers post good reviews. To eliminate concerns the best thing to do is to comply 100% with the baggage rules.

We get questions like this all the time. Basically, do the airlines follow their published rules?

I'll give you analogy. Yesterday, I drove 90 mph on the highway and didn't get a ticket. Tomorrow, you drive 90 mph on the same road. Will you get a ticket?

The answer: nobody knows.

The same holds true for carry on luggage. One hundred people say they have never had the size of their bag checked. When you get to the gate, you might have a gate agent who is a stickler for rules

Your friend has three choices:

1) Try to take the bag onboard. You may be fine.

2) Check the bag.

3) Get a smaller bag.

A few years ago my mother and I where flying to Frankfurt. The flight was full, so the staff began walking down the line for boarding. If a carry-on bag looked big, they took it, placed it in the size bin, if it was too large they tagged it for check-in. Some folks were upset, but the staff just reminded them that their bag was over the size limit. Free entertainment in line watching people have fits.

Have your friend pack into a bag that is within the limits - always pack for whatever flight has the strictest limits for bag size; you don't want to chance they'll notice the extra inch and insist not only on a gate check, but paying for gate checking the too large bag. Pay for priority boarding if you fear you will not be on the plane in the first three groups since it is the later groups boarding that tend to have their bags gate checked, even when they're within the allowed size - the plane simply runs out of bin space since there is not enough bin space for every passenger if every passenger brings a carry-on that doesn't fit under their seat.

I have flown a few Aer Lingus flights lately. I did not see them check the size of luggage, but they were weighing every bag that wasn’t a backpack. This was on flights within Europe. I saw a few people that had bags that were too heavy that they checked.