Following on from Kris Commons' second-minute opener for Celtic in last weekend's Scottish Cup semi-final, United boss McNamara saw his side fall behind to another second-minute strike on Friday night when Michael Higdon fired in off the post from 25 yards.
Higdon's opener was soon followed by a Henrik Ojamaa effort in the eighth minute and Darren Randolph protected the lead before Higdon struck again, before Jon Daly pulled one back in a spirited response from the hosts
McNamara said: "The first one was a good strike and you can't take anything away from that but we maybe could have stopped the shot.
"The second one was disappointing, from a throw-in, and he ran right through. It wasn't good defending.
"We battled hard and had a few chances where their keeper has done well.
"I knew the next goal would be crucial and I'm disappointed it wasn't us that got it."