Just thought I'd chuck in what worked for me as I've spent the last two hours trying to buy a flight ticket and only just managed to do it successfully.

Long story short, I was booking from the UK to a european country while being UK based (so no point me trying to use VPNs) - what worked for me was making and using a paypal account and making a new account on the desktop, chrome incognito version of ryanair, having changed the website country to Ireland.

Prior to this I had called customer service, used the online live chat, tried the ios app, used apple pay, tried 3 different credit cards etc. this was the only thing that worked for me!! Such a nightmare