I just got Cork-Oslo for €220 with KLM. There is value to be had out there.
I suspect one of the problems with codeshares is that people don't use travel agents any more and don't know to check the KLM and BA websites. The default second option is to check the Aer Lingus and Ryanair price out of Dublin and Shannon.
Surely the main reason for the drop though is Wizzair leaving.
Incidentally, there has actually been a decrease in frequency of buses to Dublin Airport in the last year. More than likely this is because the train service has got faster and now holds more of an advantage over the bus for passengers bound for Dublin City, but there isn't the astronomical growth that some seem to believe.