I will be flying RyanAir from London (Stansed) to Belfast (BFS) for ($17 US). According to the RyanAir website, the maximum depth permitted for a carry-on bag is 20 cm., which equals 7.8 inches. All the other dimensions are in line with a standard rolling carry-on bag in the U.S. But all of my standard rolling bags have a depth of 9”.

Has anyone traveled on RyanAir with a U.S. standard-sized carry-on bag?

I've never flown Ryanair. Based on reports I've heard from friends who have, I'd be prepared to pay their fee. The reason they're able to offer a $17 (!) fare is all the add-on fees they charge. From what I understand, they enforce them strictly.

I have flown Ryanair with my Lowe Alpine TT40 carryon which is 25cm deep and have never had any issues.

I gather you don't want to pay about US$20 to check a bag? I think it's just your gamble, you know your bags don't fit the requirement but it is true that on many airlines, they don't measure them that closely if it doesn't look ridiculous. I think you are only betting about $20, because having to pay at checkin may cost about double what it costs if you pay in advance on Ryanair.

I agree with Christina. It's a crap shoot as to whether they catch you out, but you'll definitely pay more at check-in. How much do you like to gamble ?