Catalans responded with three quickfire second-half tries to level the scores but the hosts thought Marc Sneyd won it with a drop-goal three minutes from time.
However, Folau produced a superb catch to regain possession from a short Catalans kick-off, which resulted in fellow Australian James Maloney crashing over for the match-winning score to the delight of boss McNamara.
“To come up with the short kick off and for Izzy to go up there and take it one-handed and it gives us a chance to win it, we are obviously happy with the end result,” McNamara said.
“He is a special player and it was a special catch. I thought we under-used him during the game, I thought we could have used him more.
“I thought he was solid today without being spectacular. But he comes out with a really good play to give us a last chance.
“Without a doubt, he is a real talent. We didn’t give him any area today, we never kicked the ball to him but he worked hard for the team, which is formidable.
“He is more of an attacking threat and I was disappointed that we didn’t kick the ball for him more – we never got that right.”
McNamara admitted he did not see his team winning when they trailed by 18 points with 22 minutes remaining,
Hull head coach Lee Radford was gutted to watch his team squander their sizeable second-half lead.
“I thought we did enough to win the game, probably twice. It’s heart-breaking in that manner of the last play of the game as well,” Radford said.
“I think they got a bit of wind in their sails and we didn’t break the back of that. That is something we have to get better at.
“Today I thought we did enough to get the result. It’s sort of two steps forward and one step back.”
When asked if you can coach against Folau’s late catch, Radford added: “You can, but unfortunately we don’t have anyone who is 8’2″ in our team and anyone who can jump that high in our team, so it’s very hard to replicate.”