Yes, Twin Jet has been operating for a number of years now. It is not a substantial operation, carrying fewer than 200 passengers a month. I would imagine that it is supported mainly by business passengers, hence the high fares.
On a separate note, there are some summer 2007 schedules available. Atlantic Express has some schedule changes as follows:
AEX21 JER STN 07:30 08:40
AEX22 STN JER 09:10 10:20
AEX23 JER STN 12:20 13:30
AEX24 STN JER 14:00 15:10
AEX25 JER STN 17:00 18:10
AEX26 STN JER 18:40 19:50
The midday flight seems to be operating only on Su.M.F.Sa, but changing the morning and evening flights by 30 minutes/1 hour does make Atlantic Express a more convenient option.
Moreover, Aer Lingus will be operating 3x weekly from the end of May - the schedule normally begins at the end of March.