Fitzpatrick was the eldest son and heir of Barnaby Fitzpatrick, 1st Baron Upper Ossory and Margaret, eldest daughter of Piers Butler, 8th Earl of Ormond. He was born in Ireland, probably about 1535. Sent at an early age into England as a pledge of his father's loyalty, he was educated at court, where he became the closest companion of Edward, Prince of Wales, later Edward VI, with whom he was to remain on close terms until the death of the latter. He was among the chief mourners at the funeral of King Henry VIII, the father of Edward. On 15 August 1551 he and Sir Robert Dudley were sworn two of the six gentlemen of the King Edward's privy chamber.
In France
Edward, who continued to take a kindly interest in Barnaby, sent him the same year into France in order to perfect his education, sagely advising him to "behave himself honestly, more following the company of gentlemen, than pressing into the company of the ladies there". An amused Fitzpatrick replied "You make me think the care you take for me is more fatherly than friendly". Introduced by the Lord Admiral, Lord Clinton, to Henry II, he was by him appointed a Gentleman of the Chamber, in which position he had favorable opportunities for observing the course of French politics. On his departure on 9 December 1552 he was warmly commended for his conduct by Henry himself and the ConstableMontmorency During his residence in France Edward VI continued to correspond regularly with him
In England
On his return to England Fitzpatrick took an active part in the suppression of Wyatt's rebellion. The same year, as transcribed in the Chronicle of Queen Jane by Nichols that "the Erle of Ormonde, Sir Courteney Knight, and Mr. Barnaby fell out in the night with a certayn priest in the streate, whose parte a gentyllman comyng by chance took, and so they fell by the eares; so that Barnabye was hurte. The morrowe they were ledd by the ii sheryves to the counter in the Pultry, where they remained daies".
In Ireland
Shortly afterwards Fitzpatrick went into Ireland with the Earl of Kildare and Brian O'Conor Faly,. It is stated both by Collins and Lodge that he was in 1558 present at the Siege of Leith, and that he was there knighted by the Duke of Norfolk; but for this there appears to be no authority. He sat in the Parliament of Ireland of 1559. In 1566 he was knighted by Sir Henry Sidney, who seems to have held him in high estimation. His proceedings against Edmund Butler for complicity with Fitzmaurice in the Desmond Rebellions were deeply resented by Thomas Butler, 10th Earl of Ormond, and led to a lifelong feud between them. In 1573 the Grace family, who were at odds with Fitzpatrick, abducted his wife and daughter and Fitzpatrick suspected that Ormond was behind the abduction. Fitzpatrick appealed to Sidney to intervene on his behalf, but employed the notorious felon Piers Grace to rescue his daughter. Although his wife was returned unharmed, Fitzpatrick and his brothers retaliated by spoiling the Earl of Ormond's lands. The feud between the Fitzpatricks and the Graces continued into the next century: in 1602 Richard Grace, a relative of Piers Grace, murdered Barnaby's grandson, John Butler of Dunboyne. In 1574 the Earl of Ormond made fresh allegations against Fitzpatrick's loyalty, and he was summoned to Dublin to answer before the council, where he successfully acquitted himself. In 1576 he succeeded his father, who had long been impotent, as Baron Upper Ossory, and two years afterwards had the satisfaction of killing the great rebel Rory O'More. Owing to a series of charges preferred against him by Ormond, who declared that there was "not a naughtier or more dangerous man in Ireland than the baron of Upper Ossory", Fitzpatrick and his wife were on 14 January 1581 committed to Dublin Castle. There was, however, "nothing to touch him", he being in Sir Henry Wallop's opinion "as sound a man to her majesty as any of his nation".
Fitzpatrick seems to have been suddenly taken ill, and on 11 September 1581 he died in the house of William Kelly, surgeon, Dublin, at two o'clock in the afternoon. He was, said Sir Henry Sidney, "the most sufficient man in counsel and action for the war that ever I found of that country birth; great pity it was of his death". Upon Fitzpatrick's death his estates and title passed to his brother Florence Fitzpatrick.
