The Dane came on after 74 minutes at Portman Road and played a key role in fellow substitute Dango Ouattara’s winning goal.
Billing picked the ball up in the centre, midway through the Ipswich half.
He played the ball to Antoine Semenyo on the edge of the box on the right, before getting the ball back from the Ghanaian as he shaped to shoot, creating space for Billing’s overlapping run on the right.
The 28-year-old then squared the ball across the box, to David Brooks, with his shot saved by Arijanet Muric in the Town goal, before Ouattara smashed home the rebound.
Billing, speaking to Premier League Productions after the game, said on Iraola’s instructions coming off the bench: “It was just obviously chasing a goal and getting that pretty much.
“Just challenge and hopefully bring some physicality and a bit of quality in the final third, and I think I didn’t quite get the assist but I was involved in the goal so I am happy about that.”
_(Image: Bradley Collyer/PA Wire)_ On the game as a whole, Billing said it was tough and Ipswich are not easy to beat at home.
He said the comeback shows the character of the squad, that they keep going to the end.
“I think the mentality of the whole group and the changing room is great,” Billing said.
“Just have to keep pushing, obviously every player knows what the manager wants. I think that's shown in the games. So, who knows where we can go.
“For now we just have to take one game at a time and keep trying to get the points on board and we're doing that so far.”
The Danish international said the goals will do a lot for the confidence of Ouattara and Enes Unal, who scored the equaliser.
_(Image: Bradley Collyer/PA Wire)_ “Dango is still young, but he's got so much quality and I think when he's on it, he's athletic, a good dribbler. I think he got the second goal and an assist as well, I think,” Billing said.
“It will be great for his confidence and the same for Unal, when you go from not playing so much and coming on every game and have to train week in, week out, sometimes it can be hard, mentally.
“But they've done well.”
Billing said they knew they would get chances in the game and that the comeback to keep coming even when they weren’t converting was testament to the team.
He said: “We have a lot of chances throughout the game and if we can keep going and keep pushing, eventually one is going to go in and all it takes, when they are 1-0 up, it's going to get to 1-1 and anything can happen and I think we showed that today.”