The only way it would be practical for IE to run an airport spur would be if it was in combination with building the Interconnector and 4-tracking the relevant bits of the northern line.
I think Malahide to Killester is eminently feasible (though would require quite a bit of CPOing around Kilbarick). However, Fairview to Docklands would be highly problematic. I was thinking one solution would be to extend the northern end of the Interconnector under East Wall and Clontarf and have the tunnels emerge on the edge of Clontarf Golf Course. That way you could have a station actually serving East Point and do away with the ridiculous shuttle bus.
Of course it will never happen, but since that's the subject of this thread, I thought this would be a good place to bring up the idea.