The 2011 Aviva Premiership champions launched their title challenge with a bonus-point victory that saw Mako and Chris Ashton cross after hooker Jamie George had scored two first-half tries.
While George confirmed his status as a hooker of rich promise, it was the tackle-busting Vunipola brothers that led the assault.
"You can see their quality on the ball and in that second half things opened up a little bit," McCall said.
"You give Billy and Mako time on the ball and things happen around them.
"Mako is a decision maker on the ball - when to offload and when not to, he never makes errors."
The England forwards were playing their first game together for Saracens after Billy, making his debut for the club, left Wasps during the summer to join elder brother Mako at Allianz Park.
Since returning from the summer tour to Argentina where he won both of his caps, Billy has been on a strict conditioning program that has seen him shed body fat and pack on lean muscle.
"Billy was great out there, part of a good back row. He can do very well for us and will play number eight regularly, but he can play a few positions," McCall said.
"The one thing Billy has done since he's come to the club is learn to work hard. He's lost some weight over a short period of time, the type of weight you want to lose.
"He's in the best shape he's ever been in and that's transferred on to the pitch. Hopefully his work ethic will continue.
"Billy has spent his whole life running over and through people at his own age group. To go to the next level he must work hard."
Irish's highlight in an otherwise challenging afternoon was supplied by Marlon Yarde's second-half try.
Yarde built on his fine summer tour to Argentina by cutting short treatment for cramp to join the end of a backs move that finished with him touching down.
Exiles director of rugby Brian Smith felt he was the most dangerous wing on the pitch, overshadowing Chris Ashton - his rival for England's number 14 jersey who was lined up opposite.
"Marlon's performance speaks volumes. He was probably the standout winger and is a very, very good player," Smith said.
"If he keeps his feet on the ground and keeps working hard...Marlon's biggest strength is his massive desire to improve.
"He has guys around him in the changing room who can help him go through the transition from rookie to actually being on the radar.
"There's no doubt he's a very good player with a massive future ahead of him."