Some of the RPSI ones are ex Gatwick but not all of them. They have yet to use any of the ex Gatwick stock and continue to use their Mk2A/B/D set. This set is a mixture of former BR, NIR and IE coaches
IE have had ex BR Mk1/2A/3/3B Int stock in the past and NIR have had Mk2A/B/F stock
IE disposed of the Mk3 fleet due to the leadership of their then CEO, a certain Richard Fearn formerly of BR, South Eastern Trains and Railtrack, who pushed for the 'Railcar Revolution' meaning that aside from Belfast and Cork expresses all long distance services are now in the hands of 22k railcar sets, which from a passenger point of view are excellent.
I very much doubt the 4 remaining ex Gatwick MK2's NIR have will return, laid up to rot after an expensive refurb, including the Mk2 DBSO which after £400k spent, never turned a wheel in service was quietly donated to the Downpatrick preserved line. Also the generator Mk2 for that set is now with the RPSI. Given NIR have no other Mk2 stock left, these 4 will be disposed of soon, either to the RPSI or Downpatrick
Its not the first time UK DMU sets have been rumoured to be exported to Ireland, there was talk over 10 years ago that surplus Class 222 sets would be used here but it never happened