Ryanair doesn’t overbook. That’s mainly an American thing. It does happen elsewhere, but it’s nowhere near as common.
I had to wait at the gate in the hopes there would be a no show. I wasn't the only one waiting, either. There were two of us. "Good news", they told us after everyone else had boarded. "We have seats for both of you."
They hand-wrote me a ticket and I walked onto the plane, only to find that the seat they'd assigned me was already taken. I stood in the aisle for about 10 minutes, clutching my bags, the only person still standing, until a flight attendant came and told me there wasn't actually a seat for me and that I needed to leave the plane.