It's highly unlikely to be true. Even if it was it would be extremely surprising. Given how many Irish are involved in the recruitment process in Ryanair. Previously it was a positive advantage to be Irish or indeed British for all the obvious reasons.
I wonder if this idea grows from the fact that there are very few Irish pilots coming down the tracks. This is after all a small country and with the recession there must be fewer in a position to buy a job in Ryanair.
I do know Irish pilots who are puzzled why they don't even get a call after applying. Yet they see ongoing recruitment. Strange indeed are the ways of Ryanair! Could that be the source of the rumour?
What possible reason could there be for a ban on hiring Irish? Would that include other countries, Belgium, Estonians or French, well maybe that's understandable
If your source says it's happening ask why?