Larmour scored a hat-trick in the 54-7 demolition of Italy and beautifully set up McGrath’s try.
And McGrath told Sky Sports: “He’s a freak. I get the opportunity to see him do that the whole time in training.
“Some of his tries, the one at the end was incredible. He set me up for one as well, he can do it all.
“He’s a player… it’s scary to think how young he is and the potential he could fulfil.”
McGrath also praised Larmour for his support-running capabilities.
“He’s getting more confidence with more game time,” McGrath added. “He’s like a constant battery. He’s got so much energy, he can’t stop moving.
“You look at him on the pitch and he is like that, he always wants to get the ball. He’s always talking, to the scrum-half and the 10.
“He wants to get the ball in his hands all the time. That’s incredibly exciting and it’s great to play with because it gives you options.
“He’s getting better and better and it’s going to be exciting to see where he goes.”