His claims were challenged in 1999 by The Sunday Times, which had conducted an investigation of his ancestry and found that his father was an ordinary working man in Belfast.
Later that year, recognition of MacCarthy was withdrawn and he resigned the title; in 2003 the government discontinued the practice of granting courtesy honours to claimed chiefs of the name of Irish families.
He led an affiliation of MacCarthy clan associations in Ireland, Canada, and the United States, which appealed to heritage tourism trends of the time.
His critics, pointing to his falsified ancestry, alleged that he was an impostor who misused his genealogical skills to fraudulently claim the title, then exploited it for personal financial gain and aggrandisement.
[citation needed] Investigation of the case was rendered more difficult due to the refusal of the Genealogical Office to release all documents relating to the 1992 courtesy recognition.
The Irish Freedom of Information Act of 1997 does not apply retrospectively, but documents relating to the case from April 1998 onwards were released.
Sean J. Murphy, a County Wicklow genealogist, has published online accounts of the MacCarthy Mór case and also a full-length book.
[5][better source needed] On 9 October 1999, after making no substantive answer to the "Atlanta Group", and thus losing the support of the Niadh Nask, MacCarthy abdicated the title.