Martin O'Neill has called former Sunderland boss Paolo Di Canio a "managerial charlatan" after the Italian's criticism of his tenure at the Stadium of Light.
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Di Canio was critical of the fitness levels of the squad he inherited from O'Neill, who was sacked as Sunderland manager in March.
Despite going on to ensure that the Black Cats avoided relegation last season, Di Canio parted company with the club in September after a string of poor results.
O'Neill was asked about Di Canio ahead of his first match in charge of Republic of Ireland, against Latvia, and the Northern Irishman showed he was angered by the Italian's claims.
"Paolo Di Canio? That managerial charlatan -- absolutely, yes," O'Neill said in the Guardian.
"Paolo stepped in there and basically, as weeks ran on, he ran out of excuses. I had a wry smile to myself."
O'Neill continued: "It's like a 27-year-old manager stepping in and the first thing you do is criticise the fitness of the team beforehand. If you've ever seen Aston Villa play, you'll see the one thing I pride myself on is teams being fit.
"What you'll find interesting is that, when he started, the team wasn't fit for the Chelsea game. Then the following week, when he won at Newcastle, not being fit wasn't mentioned.
"Then about two weeks later they got mauled by Aston Villa, and someone asked him about the fitness. Suddenly, he didn't know where to go. Because the team, as it progresses, should be getting more fit.
"And then, at the start of the season, when he lost by a late goal at Southampton, he was asked about the fitness regime, that he was going to have them the fittest team in the league. Suddenly, the fitness wasn't for that game but for Christmas, when the winter months set in. You know, I did have a wry smile at that one."