18:30 or 19:00 rush hour commuter flight from London to Dublin allows enough time for pax to travel out from London Liverpool St station.
Skywork's experience of Southend has shown that London City has nothing to fear from Southend as City gents who won't be paying for their own tickets don't want a 50 minute hike on a commuter line using clapped out old trains that stop at every station and where it is often standing room only when there is an airport right in the heart of the action.
There is a market for Dublin as has already been seen but that is probably more for the local area than London. Once an airline needs to attract London passengers then competition from other London airports kicks in.