“He’s the sort of man we need to see in abundance in our own party,” Mr Cowen said.
Along with many past and present members of Leinster House, Mr Cowen attended the funeral in Killarney of the former Kerry South TD.
Mr O’Leary, who had also served as a junior minister at the Department of the Environment, died this week at the age of 82.
He had been a TD from 1966 until 1997, and was a consistent poll topper.
President Michael D Higgins was represented by his aide de camp Commandant Louise Conlon and the coffin was draped in the Tricolour.
Fianna Fáil leader Micheál Martin, former justice minister Gerry Collins, and ex-ceann comhairle John O’Donoghue were among the mourners at St Mary’s Cathedral.
Mr Cowen, a close friend of Mr O’Leary, gave the graveside oration on behalf of the Fianna Fail party. He said Mr O’Leary had been a consistently decent man, a sound man, a straight man who never sought “féin-mholadh”.
He also described him as a true modern Republican. “He’s the sort of man we need to see in abundance in our own party,” Mr Cowen said, adding that, of course, the party still did have such people but needed more.
Mr Cowen said that Mr O’Leary had been “a constructive and a disciplined voice” in Fianna Fáil.
He had been a team player building the organisation and Mr Cowen hoped people would be inspired by him.
Earlier, around 400 people attended the funeral Mass concelebrated by six priests at the the cathedral.
Mementoes included a football to symbolise his love of the GAA, an autobiography — On The Doorsteps, memoirs of a long-serving TD — and a plaque to represent his political life.
Fr Niall Howard, chief celebrant, recounted Mr O’Leary’s projects: The bridge to Valentia Island, Kerry Airport, and the Department of Justice offices.
However, it was individuals and those who came to his home and his clinics who mattered most to him, Fr Howard said.
“He was never blinded by the big projects,” he said. “He knew people were not just looking for help, they were also looking for hope.”
The family home had been “his political command centre”.
He was also a deep Christian and Catholic and he understood well the saying “Níl aon uasal ná íseal, ach thuas seal ‘s thíos seal,” the priest said.
Denis O’Leary, his son, said that although his father soldiered on until 1997, he had lost the heart for politics after the sudden death of his late wife Judy in 1989.
Mr O’Leary had been particularly close to his grandchildren.
He holds a record for the longest-serving TD in Kerry South.
He is survived by seven sons and 10 grandchildren, daughters in law, nephews and nieces, and sister-in-law.