Ryanair will take your cash in Euros, but only if your first sector starts in Euroland. Also I don't know the numbers, but I will bet that buying their euros with your pounds doesn't come cheap. Probably far better to get a sterling card and try to minimise charges on that one. Of course if you spend a lot of time in Euroland it might make sense, but not, I suspect, for buying Ryanair tickets in the UK.