Former Fianna Fail TD Ned O'Keeffe was released without charge after being questioned by gardai over suspected fraud offences.
The one-time junior agriculture minister had been detained after gardai called at his home in Mitchelstown, Co Cork, at about 10.30am yesterday.
He was questioned for several hours at Cobh Garda station.
Mr O'Keeffe did not respond to questions when he walked from the station late last night.
His solicitor son Ciaran made a brief statement on behalf of his father, insisting the arrest had come as a surprise and that his father was co-operating fully with the gardai.
The former Cork East TD, who retired at the last general election, had been questioned under section 26 of the Criminal Justice (Theft and Fraud Offences) Act.
He was been held on suspicion of using a false instrument, namely an invoice to claim mobile phone expenses.
Fianna Fail refused to comment on the arrest.
A party spokesman said: "It's a matter for the gardai and we have no further comment to make at this time."
Mr O'Keeffe, 69, was a junior minister at the Department of Agriculture and Food between 1997 and 2001.
First elected to the Dail in 1982, he served as TD for his constituency for almost three decades.
He is also known as a former pig farmer and company director.