[2] Fionn was the son of Lucius William O'Brien, 15th Baron of Inchiquin and Ethel Jane Foster.

He became Aide-de-Camp to Commander British Forces Gulf in Bahrain and left the army in 1975, retiring with the rank of Captain.

[2] Although the ancestral seat of Dromoland Castle, and some of its demesne, had since left family hands, O'Brien continued to run the lands on the remaining estate.

[5] In 2012, in a long-running dispute with the management of Dromoland Castle, O'Brien was refused an initial application to the High Court for them to immediately return 37 paintings that were loaned several decades prior (after discovering the hotel had allowed damage to occur to many of the paintings, O’Brien requested their prompt return).

The hotel, however, scanned and reprinted all the paintings before O’Brien could get them returned and the copies now hang on the walls of Dromoland.