Visiting captain Gary Irvine was sent off just 20 minutes into the game for leading with an elbow in an aerial challenge with Richie Ryan.
But the Dark Blues defended valiantly and snatched the opener on 67 minutes through Ryan Conroy, although United salvaged a deserved point when Michael Gardyne drilled home a minute before time.
A relieved McNamara said: "It's a case of mixed emotions. We didn't do all that well with the 10 men.
"It was hard but we still kept going to get the equaliser. I have to praise the players for that. They never gave up.
"We had enough chances to win but it wasn't to be.
"We're still in the running for the top-six place but we need more wins at home."