League One rivals Ipswich Town make managerial decision ahead of clash against Sunderland
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The 35-year-old has signed a three-and-a-half year contract to succeed Paul Cook at the League One club after the former Wigan and Portsmouth man was sacked.
The Northern Irishman was part of Ole Gunnar Solskjaer's coaching team at Old Trafford.
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Hide AdHe stayed on after the Solskjaerwas sacked last month under new manager Ralph Rangnick
"It feels like the right time, project and club to make my first step into first-team management," said McKenna.
"Leaving a club like Manchester United was obviously a difficult decision, but I strongly believe in the opportunity to build something here."
Ipswich chief executive Mark Ashton said: "Kieran is a highly-respected young coach who has been working at the top level of English football and with one of the biggest clubs in world football for a number of years in Manchester United.
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Hide Ad"He comes to the club with pedigree within the game and with exciting plans about how he is going to take the team forward.
"When we met, there was a clear meeting of minds in terms of how we will execute the vision of this football club."
Sunderland face Ipswich Town at Portman Road on Saturday.
The Tractor Boys were knocked out of the FA Cup by League Two side Barrow on Wednesday and sacked Cook on 4 December after four months in charge, with the club in mid-table following a hectic summer transfer window that saw the club recruit heavily.