The Irish midfielder opened the scoring in the 2-1 victory at Cardiff City before team-mate Ben Watson smashed home with a long-range strike to send the Latics into the quarter-finals of the competition.
The Championship side made history last season by becoming the first team to win the FA Cup and get relegated from the top flight, after Watson scored a late winner against Manchester City at Wembley.
This term, they have taken two Premier League scalps in a row and are just two wins away from another final.
After his side's victory on Saturday, McCann said: "It's absolutely fantastic. You've seen how much it meant to everyone today - each and every player - and to our fans as well.
"We're one of the last eight left in it now and this morning we were in the last 16, so it's a great opportunity for us to defend the cup.
"We knew we had a chance coming here today and we put on a fantastic performance and we're delighted with the result."
As a Championship side fighting it out with the top guns, McCann admits Wigan have to prove their credentials but, so far, they have not been out of place. In the fourth round it was Crystal Palace who were dispatched.
"Obviously we've come down to the Championship, but it seems like we're on a level playing field," added the 26-year-old. "It was always going to be a tough day, but we had our homework done on Cardiff and it showed today.
"We played a fantastic game, we had our backs to the wall the last couple of minutes, but it was a great team performance.
"We know they have a lot of quality but we focussed on their strengths in training and we've got more than enough quality in our side to go and cause anybody problems on our day."
McCann broke the deadlock in the 19th minute when, unmarked in the box, he calmly finished off a cross from Jordi Gomez.
"I probably wasn't supposed to be up there if I'm completely honest, but I just chanced my arm and we know what kind of delivery Jordi has," he said.
Cardiff hit back through top-scorer Fraizer Campbell's low drive, before the impressive Watson made Bluebirds' debutant Juan Cala pay for giving away a free-kick.
Early in the second half Watson - who is making a habit of scoring crucial cup goals - made way for Josh McEachran.
Wigan boss Uwe Rosler later revealed Watson was carrying an injury.
Praising his fellow midfielder, McCann said: "The second goal was an unbelievable strike from Ben Watson. He's got unbelievable technical ability.
"He's got a great eye for a pass. I think he played some great passes in the first half, then capped off his display with a fantastic free kick."
The Bluebirds pressed for an equaliser late on, throwing centre-backs Cala, and captain Steven Caulker, up front when possible. It was to no avail as the visitors held on.
Ole Gunnar Solskjaer 's side must now focus on ensuring their Premier League survival.
Languishing in 19th place and three points from safety when taking goal difference into account, they have a dozen games left to prove they belong at the top table of English football.
Solskjaer has urged his players to mix hard work, belief and fearlessness to get the points they need, starting with a crucial home clash with Hull City Tigers on Saturday.
The Norwegian said: "There's talent here and we're just gelling everything together now. People have outstanding performances, but we need them now in eights or nines, not threes or fours.
"You've got to go out and play with no fear. That's the only way to play football. Not many teams stay down after a defeat - you get a bounce.
"We've responded really well before, but we will do that again."
Solskjaer was particularly pleased with "outstanding" home debutant Mats Daehli, who caused Wigan plenty of problems from midfield.
The Bluebirds may go into the Hull game without combative Chilean midfielder Gary Medel, who suffered a muscle "twinge".
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