The problem with CDG is the Ryanair flight to Beauvais. To be viable the route would need to take all existing leisure passengers from that route and add them to the people looking for onward connections.
KLM will have Aer Lingus operating 9 flights a day from Ireland to Amsterdam for them. Would going into Shannon just piss off Aer Lingus and lose them a lot more potential revenue?
Lufthansa to Frankfurt would have slot and aircraft problems, but the real problem here would be the thinness of the route as regards non-connecting passengers.