Her journey could now be coming to an end, with the production of DNA evidence to back up her claim that her nephew, Paul FitzGerald, is the rightful Duke of Leinster and should be the head of a colourful aristocratic family beset by scandal and tragedy.
The new evidence supports Mrs FitzGerald Caudill's theory that the title, Ireland's premier dukedom, has gone down the wrong branch of the family. Now there is likely to be a legal challenge to the existing holder Maurice FitzGerald, from Oxfordshire, who is currently recognised as the ninth duke.
The DNA indicates that Paul FitzGerald, a builder based in San Francisco, is descended from Lady Hermione FitzGerald, a Victorian society beauty and Leinster matriarch from whom the titled family also claims descent.
"It is an amazing story," Paul FitzGerald, 43, said yesterday. "The DNA shows that we are telling the truth."
Mrs FitzGerald Caudill, 85, from Beith in Ayrshire, claims that her father - Paul FitzGerald's grandfather - should have been given the title, but was denied it after he ran away from the family seat in Ireland to North America before the First World War.
Her father was a mysterious character, who went by several pseudonyms and led an adventurous life, working as a cowboy and a polo instructor. Mrs FitzGerald Caudill believes her father's disappearance led to the title going to his younger brother Edward, a gambler and womaniser who frittered away the family fortune.
After Edward took his own life in 1976, at the age of 83, the title descended through his son Gerald, down to his grandson Maurice, the current duke.
Mrs FitzGerald Caudill promised her father on his deathbed that she would prove his offspring had a claim to the title. Her search has led her to Scotland, where Sir Crispin Agnew of Lochnaw is acting as her family's advocate.
Under the last government, the Department of Constitutional Affairs rejected the North American claim. But it is hoped the case will be reopened after a third branch of the family agreed to take part in DNA tests. The tests show the DNA of Paul FitzGerald is an extremely close match to a descendant of Lady Hermione.
With the help of Dunfermline-based genealogist Lloyd Pitcairn, Mrs FitzGerald Caudill traced Maud Crawford, the grand-daughter of Lady Hermione's younger sister Urica Duncombe.
The results of the tests found that it was "41 times more probable" that Ms Crawford and Paul FitzGerald were extremely closely related than were from different families.The proof that Paul FitzGerald is related to the titled family is the first DNA evidence ever produced in the case, and it strongly supports Mrs Fitz-Gerald Caudhill's long-held claim suggesting that her mysterious father was the son of Lady Hermione, the wife of the fifth Duke of Leinster.
Mrs FitzGerald Caudill believes that the Leinster family froze out her father by orchestrating a bizarre change of identities, a theory supported by photographic evidence. She believes her father was christened Desmond. But when he disappeared to America, his identity as Desmond was assumed by his elder brother Maurice, who was killed in the First World War.
Meanwhile, Maurice's identity was assumed by Lady Hermione's sickly eldest son Gerald Otho, whose existence had been denied by the family and who was hidden away in the Craighouse Asylum in Edinburgh. The sickly child - now renamed Maurice -became the sixth duke, but died young in 1922.
With Mrs FitzGerald Caudill's father estranged in North America and the other son dead in the trenches, the gambler Edward succeeded to the title.
The legitimacy of the Edward line has also been questioned in a letter unearthed by Mrs Fitz-Gerald Caudill.
The letter, seen by The Scotsman, was written by Edward in 1967 to his son Gerald, the eighth duke. In it, Edward reveals that he was not, in fact, Gerald's natural father.
The letter says: "It is time you heard a few home truths - you are not my son. Your real father was a Jewish moneylender called Witkowski."
Mrs FitzGerald Caudill's lawyer, Mary McIlroy Hipwell, has written to the Leinsters' lawyers informing them of the DNA evidence.
The solicitor has threatened to "raise an action" against them unless they recognise that the North American claimant should benefit from the aristocratic family's trust.
Withers, the London-based solicitors who act for the Leinster family,refused to comment.
Mrs FitzGerald Caudill said: "I have always been 1,000 per cent sure that Daddy was the son of the fifth duke. Now there are no doubts whatsoever."