Write to Ryanair and ask for a written reason why the flight was cancelled. Ask the airline for details of the weather and why the captain decided to divert. You should also write to the Portuguese airport and ask them for details of the weather at the time the Ryanair was due to land and while it was in the area. Post back with details of what they say.
It may also be a good idea to check what the EU regs are sayng. I can't remember offhand and I'll check later.
It may well be you will have to sue Ryanair in the
county court for your money back. This particular airline are notorius for being litigious, especially for what they claim are weather related cancellation, but you would be on a stronger footing if you asked for the details and RYR refused to hand them over.