Senator Ivor Callely could sue Seanad colleagues who wrongly fined and banned him over an expenses scandal.
The politician, who quit Fianna Fail last year, successfully challenged his 20-day ban without pay from the upper house over controversial mileage claims.
A judge ruled the Seanad inquiry breached Senator Callely's constitutional right to justice and fair procedures as he was not given the opportunity to defend himself on political ethics charges.
Mr Justice Iarfhlaith O'Neill quashed the findings of a report that the politician had misrepresented his normal place of residence.
The finding opens the door to a lawsuit against the Committee on Members Interests.
Outside the Four Courts the Senator's solicitor Noel O'Hanrahan said his client had been under sustained attack but was now clearly vindicated.
"Ivor Callely, his wife and family have suffered greatly from having been places in the glare of media publicity with attendant adverse, unfair and defamatory comments from some sections of the media," said Mr O'Hanrahan.
"Senator Callely will now be reviewing his legal decisions going forward with his legal advisors and, on their advice, will not be making any further comment at this time."
The case will be mentioned a week on Monday when the issue of costs, and possibly damages, will be discussed.
Mr Callely, whose political base was Clontarf, north Dublin, claimed €80,000 for travel from his holiday home in Kilcrohane, west Cork, over three years.
The parliamentary probe, by the Select Committee on Members' Interests of Seanad Eireann last July, found the scandal-hit Senator deliberately misrepresented his normal place of residence as being Cork instead of Dublin.
Mr Justice O'Neill ruled the seven members of the Committee failed to exercise their adjudicative function by making a political judgment on the issues in the investigation, thereby breaching the applicant's constitutional rights and fair procedures.
"They misdirected themselves in law on the definition of "normal place of residence" and they breached natural justice and fair procedures in failing to have afforded the applicant a reasonable opportunity to defend himself on a charge of breach of political ethics, notwithstanding his compliance with the applicable definition of "normal place of residence", the judge added.
A barrister for the former junior minister - who was not in court for the judgment - argued the Committee had erred in applying the Department of Finance's own definition of "normal place of residence", which does not have to be a person's permanent place of residence.
Mr Justice O'Neill said while the Committee had since accepted the Senator had complied with the definition, its original determination found he had misrepresented his normal place of residence, intentionally, continually and in bad faith.
"That is what the determination says and that is how it was clearly understood in the media and by the public and that clear message comprehensively destroyed the plaintiff's good name," said Mr Justice O'Neill.
In the first case of its kind, lawyers for the committee argued the court also had no right to interfere in decisions of the Oireachtas.
However, Mr Justice O'Neill ruled as members of the public could make a complaint against a politician under the Standards in Public Office Act, it ceased to be a scheme of internal regulation, adding someone dissatisfied with the result of an inquiry would have access to the courts.
"It would seem odd, to say the least of it, if a member of the Seanad, against whom a complaint was made by a member of the public, is to be denied similar access to the courts," the judge added.
The Select Committee on Members' Interests of Seanad Eireann, headed by Senator Pat Moylan, will consider the High Court judgment in detail at a meeting next Wednesday.