I agree that Baldonnel could be a useful airfield for bizjet and other GA flights, though Weston is already serving this market to a degree and is also quite well located as regards roads etc.
While the argument about commuter flights etc. stands up to a degree, you must remember that once you separate these operations from the "mainline" Dublin airport they would have to be self-sustaining, i.e. without feed to and from international flights, which must make up a reasonable proportion of their traffic at present. Given Ireland's population and size, I would need a good deal of convincing that domestic-only operations would be viable without even greater subsidies than several of them already receive.
From an airline's point of view, it would probably have to decide whether it was going to locate all of its Dublin operations at just one of the airports, or serve both. The latter would surely drive up costs. One could certainly see a developed EIME as attractive to a carrier like Ryanair, but remember they drive a hard bargain and don't actually like paying for the development of the facilities they use. Furthermore the scale of their Dublin operations now and into the future is such that those living under the flight-path from Killiney to Baldonnel and also to the west of the airfield would need to get used to the associated noise from 0630 every morning till 2300 or later at night.
The scale of the Air Corps activity is such that it could probably stay on alongside a civil operation, though maybe circuit training might be better conducted at somewhere less busy like Shannon or Waterford.