The Irish winger is playing on-loan at the KC this season.
"We've been in dialogue with Manchester United over Robbie and we obviously want to do something and keep him at the club," Barmby told the Hull Daily Mail.
"But the thing is when you do well when you're out on loan there's obviously other clubs that will be alerted, and Manchester United can also call him back.
"That's not our decision because ultimately he's not our player, but we keep talking to Man Utd all the time.
"Robbie really enjoys it here. He loves playing with the boys and likes playing at the KC, but it's out of your hands as he's out on loan."