OK, resolved as best I can. In brief, for most routing applications there is an issue with routing to a large area, rather than a point. If an area is chosen, the application determines the "middle" of the area and takes you too the nearest point accessible from the road.

For the airport, the middle is near where the runways cross, and the nearest road to that is Matthew Hill on the W.

The workaround is explained here: [https://community.openstreetmap.org/t/specify-an-entrance-to-an-airport/119922] - basically put "Cork Airport" in the name of the terminal and car parks in the hope that routing users will select tone of those.  The changesets which resolve this comment are 157671635 and 157719071.