McDermott felt Barry illegally jumped on the back of one of his players as he leapt to reach David Silva's cross but referee Mike Dean saw nothing untoward.
Dean had been similarly unmoved when Jay Tabb went down in the area after a collision with City youngster Karim Rekik during a second half Reading counter-attack.
When asked about the winning goal, McDermott said: "Absolute joke. We have had that a few times this season and even the Tabb penalty - it was a penalty.
"We have had three or four of those, we have had a handball goal against us and an offside goal against us - all of which have cost us points.
"To get that wrong - I have just said to him (Dean) he will be gutted he's missed that. He hung in the air, but you can hang in the air all day long if you are on someone's back.
"It's an absolute cert foul, it cannot be anything else. But listen, the bottom line is we are Reading. We are a small club in this league punching above our weight.
"That will be irrelevant to you guys because Man City have won, but it is not to us. We love this club we have got, we have got a real passion. We will fight, we have 20 games to go, but we certainly deserved a draw at least."
He later added: "But just to let you know, that has just ruined my Christmas."