What the GAA's Official Guide says:
Chapter 5 -- Control of Association Property.
Rule 5.1 -- Uses of Property.
(a) All property including grounds, club houses, halls, dressing-rooms and handball alleys owned or controlled by units of the Association shall be used only for the purpose of, or in connection with, the playing of the Games controlled by the Association, and for such other purposes not in conflict with the aims and objects (sic) of the Association, that may be sanctioned from time to time by the Central Council.
(Note: Central Council shall have the power to authorise the use of Croke Park for games, other than those controlled by the Association, during a temporary period when Lansdowne Road Football Ground is closed for the proposed development).