The Tánaiste, Ms Harney, has paid tribute to her senior policy adviser, Dr Maurice Roche, who died yesterday from head injuries sustained in a horse riding accident on Friday.
Tributes were also paid by the Taoiseach, Mr Ahern, who said that Dr Roche's death had come as a grave shock to all who knew and respected him.
Dr Roche (49) had been in a coma since he fell from his horse at Brittas Bay, Co Wicklow. He had been wearing a helmet and was immediately transferred by helicopter to Tallaght Hospital, Dublin. He is survived by his wife, Eileen, and two sisters, Rita and Nell, and many nieces and nephews.
Ms Harney said Dr Roche was the brightest and sharpest person she knew, and said his death had come as a devastating blow to herself personally and to the Progressive Democrats.
"He was a trusted and loyal confidante of mine for the past nine years and I will really miss him," she said.
"He combined an outstanding intellect with a modesty and sensitivity for others that was unparalleled. His contribution to public policy in so many fields over the last decade was immense."
Mr Ahern described Dr Roche as a colleague, friend and public servant, and said the public would be poorer for his passing.
"Maurice served his country and his local community in a number of positions which are testament to his intellectual versatility," he said. "Maurice made a real difference and was a patriot of the old school, committed to doing the best for Ireland and its people."
Dr Roche joined the PDs in 1994 as chief research co-ordinator. He was appointed special adviser to Ms Harney, with responsibility for economic matters, immediately after the PDs entered government in 1997.