Cullen has been called into the Ireland senior squad for the UEFA EURO 2020 qualifiers against Gibraltar and Georgia after impressing against Portsmouth and Burton Albion.
The West Ham midfielder, on loan at Charlton Athletic, is included alongside Burnley defender Kevin Long in place of the injured duo of Ronan Curtis and Alan Browne.
And Byrne is on McCarthy’s list of potential call-ups in the future after the 22-year-old impressed for Rovers in the Tallaght Stadium win over Sligo Rovers last Friday night.
Former Ireland Under-21 captain Cullen, also 22, linked up with the Ireland squad at their team base on Sunday afternoon.
He will train with the senior squad for the first time on Monday morning and McCarthy said: “I watched Portsmouth’s Ronan Curtis against Charlton and Josh [Cullen] really grabbed my attention which was a bonus.
“He was so good that afternoon that I deliberately went to have a look at him against when Charlton played Burton on the Tuesday night. He didn’t disappoint.
“Josh is a busy midfield player. He gets around the field, he can pass the ball and he likes to have a shot. He could have scored against Burton but his own player blocked it!
“Alan Browne had a real chance of playing against Gibraltar because he’s in form and scoring goals so I’m disappointed to lose him but I had no hesitation calling Josh in after those two games.”
Former Manchester City youngster Byrne is making a name for himself in the SSE Airtricity League after moving back home and his wonder goal against Sligo capped a Man of the Match display.
Ireland manager McCarthy was at the game alongside his assistant coaches Terry Connor and Robbie Keane, as well as Under-21 boss Stephen Kenny and Under-19 Head Coach Tom Mohan.
“Jack was the star of the show in Tallaght,” said McCarthy. “He was the best player by a distance in a good team performance by Rovers and his goal was the icing on the cake.
“I was impressed with his vision, his range of passing and his passion. Robbie has trained with Jack with the senior team before and speaks highly of him so I wasn’t surprised by his contribution.
“He will definitely come into consideration for the squad in the near future. Well done to him.”
Currently awaiting an assessment of Callum O’Dowda’s knee ligament injury - the Ireland medical staff are analysing scans - McCarthy will finalise his 23-man squad on Monday morning ahead of training at the FAI National Training Centre.
Midfielder Conor Hourihane will arrives in on Monday following the birth of his daughter, along with defender Richard Keogh and goalkeeper Keiren Westwood.
“I am looking forward to getting back onto the training ground,” said McCarthy. “It’s almost a year since I left Ipswich and I have missed it.
“The honeymoon has been great since I got the Ireland job again and I have enjoyed the positivity and the support of the Irish fans everywhere I’ve gone.
“But we’re here to win matches and get Ireland to the EURO 2020 finals. That work starts on the training ground on Monday. I can’t wait.”