Section 4.1.3 of the 2010 Hilton Brand Standard mandates that if the Hilton is designated as an Airport hotel, they must provide complementary transportation to the airport unless they are adjacent to the Airport terminal.

However,
1) Hotels can seek a waiver from any of the sections of the brand standards.
2) This brand standard is out of date and this provision may no longer be in the current document.
or
3) The document defines an airport hotel as being within 3miles of the airport and Hilton DUB is close to twice that distance. So, they may not be counted as an airport Hilton.