Ivan Sproule's 53rd-minute strike proved enough to earn the points for the hosts and take them to fourth, while United had to play the last half-hour with 10 men after Barry Douglas was sent off for a dangerous tackle.
United are now seven points adrift of third place with only three games remaining, and McNamara said: "I think that is it over for Europe. Realistically, it would be very difficult for us.
"I thought it was a game we had to win and I'm very disappointed with the way it panned out, especially with the sending-off.
"The first goal is always crucial when you come here, the way they play and defend. It's difficult."
Douglas saw red despite winning the ball in a challenge on Sproule, and McNamara added: "I've just seen Barry's challenge on the monitor there and he has won the ball. I didn't even think it was a yellow, never mind a red."