There's 2 strands of thought to this - you either wheel it up to the desk with no packaging and trust that the carriers will handle it more carefully, because they can see it's a bike, and it's easy for them to handle, or else you box it and put padding around delicate parts, but risk that a heavier item could be thrown on top of it.
I think if you take a short-haul Ryanair or Aer Lingus flight you should be fine, since not many people are checking in luggage these days.