My experience FWIW. I flew to Spain last week. Bought a seat for the bass- an old Aria Precise- which was cheaper than putting it in the hold. £41 vs £50 and of course it stayed with me at all times. Shockingly straightforward, nobody batted an eyelid at security, on the plane, etc etc. There are instructions on how to book a seat for a guitar on the Ryanair website. I had no trouble with customs at all. There should be no need for a carnet with just a bass, as this comes under your 'personal belongings' allowance.