Telegraph obit below. They also use the of in the title. I wonder if the family (ie the Earl) used that style as well?
The Earl of Erne
Anglo-Irish peer who renovated Castle Crom in Co Fermanagh to which he welcomed the Queen
The 6th Earl of Erne, who has died aged 78, was for 25 years the Queen’s representative in Co Fermanagh, Northern Ireland, and the owner of Crom Castle, on the shores of Upper Lough Erne....
He was appointed Lord Lieutenant for Co Fermanagh in 1987, succeeding Viola, Dowager Duchess of Westminster. ....
he family had close links with the Royal family. His father, the 5th Earl, had served as a Page of Honour to King George V, and before his death in 1940 was a Lord-in-Waiting to George VI, who became Henry’s godfather. Lord Crichton’s aunt, the Duchess (later Dowager Duchess) of Abercorn, was a long-serving Mistress of the Robes to Queen Elizabeth the Queen Mother and Lord Erne would serve as Page of Honour to the Queen at the Coronation in 1953....
http://www.telegraph.co.uk/news/obituaries/12089668/The-Earl-of-Erne-obituary.html