United Oil & Gas plc announced that David Quirke will step down as chief financial officer (CFO) and executive director at a date to be agreed.
The company added that a search for his replacement was well advanced.
Mr Quirke, who is leaving to pursue new challenges, will remain in his post to ensure an orderly transition.
Chief executive Brian Larkin said that Mr Quirke had made an outstanding contribution to the company and that he was sorry to see him leave.