They could stop the trains at Heuston Platform 10 for regular users, then reverse back out & head through the Park Tunnel to Connolly for the footie fans, but somehow I can't imagine IE having the sense to do that...
Or just run an ACTUAL special through to Connolly...
Re. them closing Grand Canal Dock on match days, I'd imagine it's for "crowd control" reasons, like the way close to match kick off they stop the northbound DARTs at Sandymount & skip Lansdowne Road. The local residents would probably object to trains stopping at Grand Canal Dock as well...