Good catchment area for Ryanair destinations not served by others, so seems a logical choice but they would only do it if the operating costs at the airport were low enough. So although they may attract lots of passengers don't bank on the airport making much money out of it.
They may be looking quickly for alternative bases for aircraft which would otherwise have used EU airports now affected by the recent EU ruling. They need somewhere to operate the continuing new deliveries of the B738s. There is a limit to what Stansted can cope with.