TWO of the country's leading advertising companies have merged to form the third largest agency in Ireland. Quinn McDonnell Pattison/DMB&B announced yesterday that it had merged with O'Keeffe's Advertising after several months of discussions. The new firm, which will retain the name QMP/ DMB&B, will employ some 40 people and have combined billings of £17 million. Mr Ray O'Keeffe, the former manager of O'Keeffe's, is to be appointed chairman of the board.