He said Mr Costello was far more pleasant than people appreciated and helped bring Ireland forward from the lawlessness following the Civil War.
“He made reluctant democrats out of his opponents. He could not have done more and the country deserved no less,” he said.
Mr Cosgrave — taoiseach from March, 1973 to July, 1977 — was speaking at the launch of the extensive new biography of Mr Costello, who twice led inter-party governments between 1948 and 1957.
The Reluctant Taoiseach, written by RTÉ political correspondent David McCullagh, details the controversial times and tenure of Mr Costello.
Mr Cosgrave, who was among those who successfully persuaded Mr Costello to lead the 1948 government, said apart from the divisions at the time, the governments were successful.
He said the IDA was created and the conditions improved in the area of exportation taxes, which made substantial improvements in the economic situation.
And he said, in his earlier career as attorney general, Mr Costello worked to ensure Ireland was recognised at the League of Nations and had independence in its judicial system.
He was meticulous in his appointments and focused on his responsibilities, Mr Cosgrave said.
“[Mr Costello] was totally determined and never flagged in his single-minded purpose to discharge his job whatever it was.”
The launch, at Dublin’s Mansion House, was packed to capacity and attracted the majority of the Fine Gael parliamentary party.
Former taoiseach Albert Reynolds also attended along with Taoiseach Brian Cowen.
Mr McCullagh praised the openness and assistance of the Costello family for their willingness to allow so much information to be contributed to the research.
He said this should address the lack of attention given to Mr Costello’s time as taoiseach: “His modesty means he was somewhat overlooked in the history books. I hope in a small way this book will help correct that.”