A first-half Shane Long effort and late strikes from Ian Harte and Noel Hunt put the Royals in command before Connor Wickham netted a late consolation for the Tractor Boys.
McDermott was delighted with a fine display away from home by his side, who still have play-off aspirations.
"That's as good an away performance as you'll see in the Championship. We went to Burnley and played similarly to that and won 4-0," he said. "We've had some really decent away performances but drawn too many games. We've been working on that and to start turning the draws into wins is good news.
"This is a good stage of the season to find some consistency and that's what we're trying to do. There's still a load of points to play for and many things can happen."
On the FA Cup date with Manchester City at the weekend, he added: "Now we're looking forward to a good day on Sunday. City have got top, top, top players and a terrific manager too, so we'll see."
Source: PA
Source: PA