Senator Brian Ó Domhnaill has criticised the Standards in Public Office Commission (Sipo), after it found that he intentionally breached ethics legislation.
Mr Ó Domhnaill has written to Fianna Fáil TDs and Senators outlining his objections to the ruling by the body.
He also confirmed he will be appealing the decision to the High Court.
Mr Ó Domhnaill resigned from Fianna Fáil following the findings, but is facing calls to resign his Seanad seat.
The Co Donegal-based Senator said the findings had undermined and compromised his personal and professional integrity.
He said this had been done in the absence of any real evidence to support the body’s findings. He claimed this process had “completely gutted” him.
In his letter, Mr Ó Domhnaill said: “I categorically refuse to accept Sipo’s findings in every respect and will seek to vindicate my good name, by whatever means, to right the wrong that has been done to me.”
Sipo found earlier this month that Mr Ó Domhnaill intentionally broke the rules on ethics while he was a county councillor in Co Donegal a decade ago.
The commission found against him after a public inquiry into his travel and subsistence claims during this period.
The body investigated allegations that he had claimed expenses for events that were held at the same time in different parts of the country.
Sipo said each breach was committed intentionally and was “a serious matter”.
In the letter, the Senator said the findings against him pose threats to all public servants.
He said he will not stand “idly by and do nothing”, but will seek the truth and ensure the public is aware of the persons who initiated this dispute.
Mr Ó Domhnaill said: "For over five years, I have faced relentless, uninterrupted and thorough investigations by Sipo and previously, by Donegal County Council, due to a complaint, since known to be a fraudulent and unfounded complaint, made to Donegal County Council.
“As a result, the pressure during this time has been intense and all-encompassing. I wish to pay generous tribute to my wife, family, friends and colleagues who never lost faith in me.
“It is their unwavering support that has given me the strength and encouragement to cope with a very disruptive and traumatic period of my life.”