The Minister for Foreign Affairs, Dermot Ahern, has said any hard evidence that prisoners are being transferred by the US through Shannon would be investigated by the gardaí.
Mr Ahern said the gardaí and the Director of Public Prosecutions are the only ones who can enforce the law in Ireland.
The Labour Spokesman on Foreign Affairs, Michael D Higgins, said the DPP had not acted on two complaints on this issue.
Mr Ahern said he suspects this was due to a lack of evidence.
