"I just hired a great new actor for Transformers 4 to star against Mark Wahlberg," director Michael Bay wrote on his website yesterday.
Describing him as "an Irish kid that came to America with 30 bucks in his pocket", Bay said the ex-Belvedere College student was "pretty ballsy".
Reynor will play the love interest of Mark Wahlberg's daughter in the film.
Multi-award winner Bay admitted to becoming aware of Reynor after seeing him play Richard Karsen in Lenny Abrahamson's Ifta-nominated movie.
"I spotted him in a great little Irish movie What Richard Did. This kid is the real deal."