Wickham, who qualifies for the Republic through his Northern Irish father Stefan, an Army warrant officer, has scored six goals for England Under 21s in 17 appearances.
Ireland coach Martin O'Neill said: "I worked with Connor at Sunderland. Steve Bruce bought him but I thought the price tag concerned him and for a little while I thought being a professional footballer was enough for Connor and at that stage that he liked the lifestyle which was all very new to him.
"But you have to knuckle down and actually have to go and do something. In my latter days at Sunderland it looked as if he was going to come round and wanted to become a professional player."
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