Midfielder Ryan Stevenson was shown a straight red card by referee Willie Collum for a 25th-minute challenge on United winger Gary Mackay-Steven which left the Jambos facing an uphill struggle.
And goals from Johnny Russell, Willo Flood and Michael Gardyne made Michael Ngoo's 76th-minute strike a mere consolation.
But McNamara said: "I thought we were guilty of overplaying in the wrong areas at times. Sometimes we took too many touches on the ball which didn't help.
"But overall I'm happy with the result which means we've got two home wins.
"Rado Cierzniak was very commanding but unfortunately he conceded that goal and there was nothing he could do about that. The only downside was not keeping a clean sheet."
On the sending off, McNamara added: "Obviously it helped us more in terms of 11 v 10 but it didn't help the spectacle of the game.
"I thought it was harsh but I would need to see it again.
"When we passed it quickly we really hurt them. Mackay-Steven had three of their back four booked from running at them so we have good talent there and confidence."