Presidential candidate Dana Rosemary Scallon ended a live television debate with a broadside criticising a newspaper article.
The former Eurovision singer claimed that a malicious and vile allegation would be made against a member of her family in an attempt to destroy her good character.
She previously threatened to pull out for the race of the Aras because of what she called the intrusion into her family life, after details of a 1998 court case involving her and her sister were published.
Dana revealed lawyers had been called in to examine the latest claims.
"It has come to my attention that yet further allegations, this time of a most untrue, malicious and vile nature, have been levelled against a member of my family," she said on RTE's Prime Time. "Let it be known that lawyers have already been instructed to forensically investigate a particular communication disseminating this vile and false accusation, which attempts to implicate me and destroy my good character.
"We have now been advised that all possible lines of inquiry are being pursued with prosecution authorities in the United States. May I assure the Irish people that I will leave no stone unturned to expose the malicious intent at the heart of these untrue allegations."
Dana was close to tears as she read the statement at the end of the live presidential debate, which also featured independents Mary Davis, Sean Gallagher and David Norris, Fine Gael candidate Gay Mitchell, Labour's Michael D Higgins and Martin McGuinness of Sinn Fein.
The former Eurovision winner refused to elaborate on the claims, instead lashing out over media coverage of her campaign bid. "That is not the first attempt to destroy my character and for us running forward, just to stand before the people is hard enough and to answer questions," she added. "But to feel that there is a deliberate attempt to destroy your character is extremely difficult."
Last week Dana was forced to deny she covered up her dual Irish-American citizenship, a claim reportedly made by her sister Susan Stein during a court case in the US in 1998 during a bitter row over ownership of some of the singer's recordings. The Euro-sceptic former MEP revealed she took the oath of allegiance to the US in 1999.
Dana hit back stating it was a very low ebb for the media to use a painful family dispute, which has been settled, to paint her as deceiving the people of Ireland.