From my knowledge/experience of the selection process, it's conducted entirely by Ryanair staff. CAE and SAA simply select the students which are sent for assessment, maybe why on my recent trip to Stansted there were 4 from Sweeden and 2 from the UK.
EDIT: I was also told by one of the Ryanair Training Captains that the decision lies with Dublin as to who ultimately gets selected to go forward to TR.