I'm hesitant to suggest that you should 'chance it' (because I don't want to get blamed if I get it wrong!) but I've flown on several Ryanair flights where youngsters have had Trunkis, and there never seems to have been a problem.
Further, people on this thread state that they've travelled with them, on Ryanair, without being challenged over the dimensions:
http://forums.moneysa...wthread.php?t=2618391
Fliptheswitch:
When the Europe-wide security rules (relating to such things as liquids in hand baggage) came into force, there was a universal limit of just one item of hand baggage, with maximum dimensions specified by the Department of Transport (and the relevant departments of other European governments). However individual airlines were still able to have stricter rules (based, for example, upon the capacity of the overhead lockers in their aircraft).
Trunki (and other manufacturers) based their claims that items could be carried as hand baggage upon the rules set by governments, and not on those set by individual airlines. The government rules have now been removed but Trunki (et al) still use the dimensions which were contained within those rules as the basis for their claims (since they can't be expected to know the individual rules of thousands of airlines throughout the world).
Chris