Fianna Fáil Micheál Martin has said the leaking of information regarding Fine Gael TD Sean Conlan marked "a sinister turn in our politics".
Speaking in the Dáil, Mr Martin raised the recent controversy over a diamond ring.
Mr Martin said he had gone to great lengths to get information out of the Taoiseach's office through the Freedom of Information Act in terms of policy.
Yet, he said, information about private citizens could come out of the office at the drop of a hat, with serious implications for TDs or for anybody who crosses the Taoiseach.
Mr Martin said that this was a serious matter because people think they can write to the Taoiseach in confidence.
He said it represented a sinister turn in Irish politics, which has been a pattern of this Government to date.
Mr Conlan has said he was at an antiques fair in the UK and saw a ring that he subsequently took back to Dublin to be appraised.
He said he subsequently chose not to buy it and returned it to the dealer.
Taoiseach Enda Kenny was then contacted by the dealer.
Mr Conlan said he had previously pointed out that if you speak out or criticise the Taoiseach or Government policy, they will try to damage you and he believed this was an attempt to do that.
He said: "The Taoiseach received this email back in August long before the McNulty affair, why is it only surfacing now after the McNulty affair when I spoke out and criticised the Taoiseach?
"I found out from (antiques dealer) Freya (Hart) that only two people received the email, the Taoiseach and the Minister for Arts Heather Humphreys, so the position is how does an email meant for two people end up on the cover of the Irish Independent?"
Mr Conlan said he did not know why the dealer contacted the Taoiseach and described the report as a "nothing" story.