Would like to see Ryanair start up in BFS too.
Think it wouldn`t impact Derry too greatly. The distance between Derry and Belfast International airports is a similar distance between Ryanair`s 2 bases in Munster. Ryanair operate 10 routes out of Shannon and 24 out of Cork so there is room for Ryanair to have a 2nd base in the north.
Think Easyjet would hold their ground in BFS too.
Ryanair and Easyjet both operate out of Gatwick, Standsted and Luton and fly to largely different destinations.
That said the only room Ryanair have for expansion is in their winter schedule until their next order of jets starts to be added to the fleet from Sept 2014.