Almost every airline allows "a personal item" in addition to your carry-on luggage; otherwise people with purses wouldn't get any carry-on. Your poster tube is probably not "a personal item", but your carry-on could be if it's small enough to fit under your seat. (That's the real reason: there's not enough overhead bin space, but there's plenty of under-seat space. Many people hate having stuff at their feet, though, so the airline can't count on that space being used; saying that you can take a personal item that fits under the seat works for you.) In any case, the way this all works out in practice is that if you're lugging a huge amount of stuff that doesn't look like it'll fit, you'll be asked to check some at the door. Otherwise, it's all okay as long as it will either fit overhead or fit under your seat without sticking out inordinately. I've never seen problems even with backpack (personal item) + carry-on (overhead, fitting strictly within size limits and without requiring lots of shoving to get it to pretend to fit) + poster tube, except on really small regional airplanes with desperately tiny overhead bin space.
Also, if the poster tube doesn't fit in the overhead luggage--it nearly always does--they'll be able to check it at the door to the airplane, just like they do with other overly large and slightly fragile items like strollers.
And, finally, almost everywhere you will present has its own poster printing facilities within a few miles. Any institution of respectable size will have their own, and any city of respectable size--and you're rarely far from one in the EU--will have a copy store of some sort at which you can print at poster size.
So just take the item with you as carry on, and in the extraordinarily unlikely event that something goes wrong and the poster is destroyed, get it printed at your destination. (If you are extra-paranoid, you can get a somewhat tougher plastic poster tube instead of the cardboard ones--bearing in mind that I have yet to see a cardboard one get so crushed that the poster was significantly damaged.)