The statement to be read to Ryanair passengers by the captain, apparently during all diversions, is as follows:
It is prohibited by law for security reasons to allow any passengers to disembark at this airport, despite its proximity to your original destination. This situation is completely outside my control.
Notwithstanding the interesting comments and speculations since the posting of the original memo at the start of this thread I am none the wiser as to whether or not this statement is correct. It does not look to me as if it is correct because it makes a blunt statement about "control of the captain" and "legality" (and not about lack of ground services, immigration, customs, space in the terminal building, the rights of the captain in particular circumstances, etc.).
It looks as though it may really mean "It is inconvenient to Ryanair if you try to disembark at the alternate airport ..." Can anyone with some legal training comment on the general claim about "illegality" contained in the Ryanair instruction to pilots? Could it be legal for a captain on another airline's diverted aircraft to let people off at the same alternate? Also, it seems rather unlikely that only Ryanair would suffers from such a general legal impediment.