Landing at an alternate location is significantly more expensive, so I assume Ryanair put pressure on its pilots to avoid that…?

We'll find out in the investigation, but "get-there-itis" is a very common condition amongst pilots and can lead to them delaying making decisions (such as going to alternates) so it's possible that this happened without explicit (or implicit) pressure from management.

That being said, the fact that (AFAICS) they first tried to divert to a closer airport where the weather was similar rather than an alternate with clear weather was probably one of the causes of this event.