Gavin Gunning grabbed the third from the penalty spot after he was fouled by Graeme Shinnie and goals from Gary Mackay-Steven and Stuart Armstong completed the rout.
Inverness finished with nine men following straight red cards for Greg Tansey and Marley Watkins, both of whom fell foul of referee Willie Collum for studs-up challenges on Paul Paton.
"I thought we were excellent from the start," McNamara said. "Nadir took his first goal really well and showed the desire to get on the end of the second one.
"When he's on form he is unplayable. The big thing for the first one is the strength he has got, and the second one you can see the lad looking over his shoulder looking for him. He has real pace and power and I thought he was fantastic.
"We were very comfortable in the match, to a man we were excellent."
United have now won five matches in a row and McNamara sees no reason why they cannot go all the way in the competition.
"We got to the semi-final last year against Celtic and we ran them close," the former Scotland international said. "So it would be good to get to the final this year and there is no reason why we can't go on and win it.
"That's five games we have won on the bounce. That breeds confidence in the
players and we are scoring goals as well."
Inverness boss John Hughes refused to accept that Tansey and Watkins deserved their red cards.
"A sending-off for me is going over the top of the ball, when you go to damage," Hughes said. "Tansey has put his studs round the back of the ball, the guy has kicked it and cleared the ball and went down like a ton of bricks. He is up again running about as soon as Tansey is sent off.
"Then Marley slide-tackles, puts his studs behind the ball, a great tackle in my day. I don't know what justifies red cards these days.
"Take nothing away from Dundee United but we beat ourselves. The first two goals were very avoidable and the second one was comical.
"It was a penalty and we were beat.
"I think the sendings-off were irrelevant. I don't know what justifies a sending-off at this moment in time but we were 3-0 down before the first guy gets sent off."