I’m going for a short fly with my uni. 27.5".
I have a bicycle bag and will stuff it with most of my other things (clothes and so on) since I don’t have a suitcase.
I had to opt-in for the 20 Kg because of the dimensions though.
Did anyone fly with Ryanair and a unicycle? Did they encounter any issues?
I went with Air Berlin some years ago
It was no problem as long as i dont say the Cycle word. 
If they ask you say that is circus stuff, because any Bicycle and also Monocyle will have extra charge
I also did it, with wheel bag and 27,5" no issues. Since measurments are kind of weird, they might tell you when checking in you baggage to bring it up to the over sized counter while it still accounts for a normal bag. And the tire must be deflated.