Dunne, the former boss of failed concrete contractor Dunne Group, joined Keltbray two years ago when the assets of his concrete firm were bought by Keltbray from the administrators.
He was involved in setting up Keltbray Structures and is understood to have taken a 20% stake in the business at the time.
In the interim, Andy McClafferty, Keltbray Structures COO, will assume the role of MD.
A Keltbray spokeswoman said: “We wish to announce that Keltbray has accepted the resignation of Gordon Dunne, MD of Keltbray Structures. Mr Dunne will now be on gardening leave until the expiry of his notice period. ”