Later this year I plan to take my Trek 1200 road bike by air from
Ireland to Italy and home again. I will be travelling with Aer Lingus,
Ryanair and Easyjet.

Each of their websites stipulate that one must carry a bike in a
bicycle box or bike bag.
I am aware of the moulded plastic bike boxes that can be used to carry
bikes. Would any or all of these airlines allow one to carry a bike
in the heavy-duty cardboard box in which it was originally delivered
to the bike shop?

Cheers
SB