A Cityjet Avro RJ-85 on behalf of Air France, registration EI-RJY performing flight AF-1016 from Paris Charles de Gaulle (France) to Dublin (Ireland), was enroute at FL300 about 60nm south of London's Stansted Airport,EN (UK) when the crew decided to divert to Stansted reporting problems with the cabin pressure. The aircraft descended normally and landed safely on Stansted's runway 22 about 30 minutes later. Stansted Airport reported a mechanical issue related to "decompression".