The SPL rules state that three players in a club's 18-man squad must be under-21 which has left McGinn, 22, on the outside since signing from Derry City in January.
The Northern Ireland international, however, was handed his debut by Tony Mowbray in the 1-1 draw with Dundee United at Parkhead as replacement for suspended Aiden McGeady.
Tannadice striker David Goodwillie opened the scoring in the sixth minute before Hoops hit-man Scott McDonald equalised soon after but United, who had midfielder Scott Robertson sent off near the end for picking up the second of his two yellow cards with a foul on Celtic substitute Georgios Samaras, deserved their point.
After his man-of-the-match performance, McGinn said: "I was nervous at the start but hopefully I have grabbed my chance.
"If I am in the squad for the Europa League game against Hapoel Tel Aviv on Thursday then I will be happy. If I get the nod to start I will be over the moon.
"But Maloney and McGeady are playing frightening stuff at the minute and it's going to be hard.
"The under-21 rule is going to kill me for the league games but any chance I get to play I will grab it with both hands.
"It was my first SPL game and thank God it was at Celtic Park, the fans took to me and they gave me momentum.
"It's been frustrating at times, it's only my second year as a full-time footballer but I will keep working hard."
Mowbray could not offer the youngster from Donaghmore much succour, saying: "It shouldn't be a surprise. He is one of these unfortunate lads who is not under 21 but has bags of talent.
"He finds himself behind McGeady and Maloney and most weeks doesn't even get on the bench.
"It would be nice to sometimes take Aiden off and put Niall on but you don't get the opportunity.
"So he has been unfortunate he hasn't played more football."
Samaras had a goal chalked off in the second half for offside and the Hoops looked to be denied a late penalty when United defender Darren Dods appeared to use his hand to block an Andreas Hinkel cross.
Mowbray refused to elaborate on either decision but hinted that the goal might have stood.
"I don't think either team deserved to win but if Samaras' goal had been allowed to stand I would have said we deserved to win, it's fine margins," he said.
"The linesman put his flag up so there's no point in me having a moan about it, watch it on the telly and make your own mind up.
"I thought my team put in a fantastic shift. But we played against a decent team and we have to accept the point."
United manager Craig Levein claims Dods should not have been penalised for hand ball when trying to block Scott Brown's cross, which led to the free-kick from which McDonald levelled.
He said: "I had a great view of it. I watch football on television 24/7 and nowadays that is not a free-kick.
"If Darren Dods is eight to 10 yards away and puts his hand up to try and stop the ball then I can accept that but he was too close to the ball to have been judged to deliberately handle it.
"And I've yet to see anybody who can jump to try and block a cross with his hands by their side"
Levein, however, made a joke of Dods' apparent hand ball late in the game, saying: "Darren said he was really disappointed that he didn't hold it.
"But I can't argue with the sending off. Scott had three or fouls and had been booked and was booked again. But there was another player who had 10 fouls today - not from my team I may add."
Dods insisted that luck evened itself out over the 90 minutes.
"It's hit my head then hit my thumb and bounced away but I think we were unfortunate in the first half," he said.
"I had my hands by my side and the referee didn't give it but the linesman did.
"So we were unfortunate there and some people have said it looked like a penalty when Danny Fox tackled Goodwillie in the box so these things even themselves out."