Luggage restrictions - how strict?

Have you looked on the Rick Steves Europe store? Possibly they'll have something meeting your needs.

I have, and there's nothing there that meets my needs unfortunately. Can't be a backpack, and I don't want wheels, etc. Ideally I'd like to know whether I can bring my 57cm duffel onboard from someone who's done it (or found out the hard way that you can't), or failing that, which products others are using that are acceptable on the European ULCC airlines with extreme weight and size restrictions.

I have a 20.5" Travelpro spinner that I can use in a pinch but it restricts what I can bring back just a bit too much. Fine for 3 nights, not for 6. A true well-built 20" canvas duffel would be awesome.

Your two measurements are almost exactly the same: 55 x 40 x 23 cm is equivalent to 21.6" x 15.7" x 9.1", which is almost exactly the same as your first set of measurements of 22" x 14" x 9" (= 55.9 x 35.6 x 22.3 cm).

Norwegian states their measurements in cm, but these measurements are very similar to Aer Lingus (55 x 40 x 24 cm) and American/United/Delta, which state their measurements in inches (22" x 14" x 9").

If your bag fits a 22" x 14" x 9" specification, it should fit the 55 x 40 x 23 cm specification unless it's really right on the edge. Are you sure you are converting correctly?

Out of all airlines, I've found Norwegian Air Shuttle to be tighter on baggage in European airports than any other airline I've ever flown. Once they singled my carry on for being overweight so I offloaded enough stuff into my wife's bag to get under--holding up their line. Another time (Oslo), they made me put my bag in The Rack. My 21 inch swivel wheel bag wouldn't go into The Rack because of the 1' wheels. They hit me up for $50 and the bag got checked. On the plane, over a hundred people had already passed thru the line with larger bags than mine.
I have not ever had any problems with the same baggage getting on a plane in the U.S.
Last year, I switched over to a 21" carryon backpack suitcase--with no issues as I was having to also handle my wife's luggage since she was temporarily in a wheelchair.

The other two dimensions of the duffel bag I bought are well within Norwegian's specs (with room to spare), but the length is very definitely a couple of ticks over 55cm when full, because the sides spill out a little bit due to the bag being soft-sized and thus a bit expandable.

I think it would scrunch up a bit if they made me stick it in the rack, but considering their at-the-gate checking prices are $100 outbound and €100 on the return, I'm hesitant to risk it. It's too bad, because I really like the bag. It looks great and has all the pockets I need, but that one dimension is troubling.

As long as you are measuring including the wheels, you should be okay if it squishes up. Be careful with the weight limit. While I haven't seen agents (Not Norwegian - which I haven't flown) checking sizes using the size rack, I have seen them weighing carry-on bags at check-in.

I'm well under (6-7kg, tops), and the bag doesn't have wheels. It's just a plain-ol' canvas duffel bag with two carrying straps at the top. If they get finicky and make me put it in The Rack, it would probably squish down OK, but I'm wondering if that would be enough. Like I said before, I also have a Travelpro 20.5" spinner, but it holds far less and wouldn't allow me to bring anything back. Which is why I really like duffels.

A soft sided bag that isn't stuffed completely full will work. It will squish slightly to fit the required dimensions.

I think all of the northern Europe air carriers are strict on baggage. I've only flown SAS and Lufthanse in that category. Both at least looked at everyone's bag, measured/weighed the ones that looked close and required those with obviously stuffed bags to check them.