The Supreme Court has reserved judgment in an appeal against a High Court decision that found the Committee on Members' Interests had acted beyond its powers in suspending Ivor Callely from the Seanad in 2010.
Three years ago, the Seanad Committee on Members' Interests suspended Mr Callely for 20 days after finding he had misrepresented his normal place of residence as Cork for expenses claims.
Mr Callely challenged the decision in the High Court, which found in his favour.
The court said the committee had acted beyond its powers in its findings against him.
The committee argues its actions fell within parliamentary privilege and the courts have no power to review its actions.
Lawyers for the former senator argued that the committee operates under Ethics in Public Office legislation, which can be reviewed by the courts.
Michael O'Higgins, Senior Counsel for Mr Callely, told the seven-judge Supreme Court the committee operated under a "public act which governs the relationship between the Oireachtas and the public".
"It is not a private charter," he said.
However, counsel for the committee Conleth Bradley said it was clear that parliamentary privilege provided for by the Constitution could be applied and the Seanad had dealt with the matter appropriately.
The court will give its judgment at a later date.