The 'rules' state that they're supposed to ask for volunteers, but in this instance, they didn't. I was just withheld from boarding until they confirmed they had a seat for me.
(How it happens can vary: your PNR might not have a seat assigned, or the seat might have been double booked. Either way, at check-in they’ll typically notify you and let you through regardless, since you have a perfectly valid ticket.)
I still had a valid boarding pass. Nothing went wrong at the airport, besides Ryanair failing to follow correct overbooking procedure. Fortunately in that instance, had they refused boarding on me, I would have suffered zero inconvenience, and have received 350 euros compensation and a flight the next day. In a way, I wish they had refused me that day.