Clearly there is demand for it...there wouldn`t be an hourly train service between Cork and Dublin if there wasn`t demand for it!!! And that`s competing with at least 2 or 3 bus companies on the Cork-Dublin route!!!
If some trains from Cork were routed through the Phoenix Park tunnel and onwards via a spur to Dublin airport and reconnecting north of Malahide to the Belfast line, pax from not just Cork and Belfast would have direct access to Dublin airport but stations along the whole national rail network as well as Dublin commuter/DART trains would have access to Dublin airport.