Brían, finding his brother Diarmaid an embarrassment and a hindrance to his ambitions of becoming an English Baron, gave up his brother to the Butlers and the Butlers acted out their cruel revenge on him. That being done, Brían, in 1537, submitted to the English king and gave up all his ancient rights. So in 1541, Brían became Barnaby Fitzpatrick, the First Baron of Upper Ossory at Castletown.^[3]

He was afterwards imprisoned at Waterford until he restored "some preys he had seized in Leix".^[4]

First, about 1491 he married an unknown daughter of Ó Mórdha, who was his first cousin;^[5] she died, probably in childbirth.^[6]

Second, about 1492 he married an unknown first cousin, who was also the first cousin of his first wife. Without dispensation from the church this second marriage was declared null.^[5]

Third, about 1493 he married Catherine Ó Mórdha, who was his half-sister and also the half-sister of his first wife. They had children, which resulted in an uproar among the relatives of the second wife. Nevertheless the Pope absolved the pair, permitted their marriage, and decreed their present and future children were legitimate.^[5] According to Carrigan^[7] Brian’s children before his marriage to Margaret Butler probably included:

• Teige, who his father, Brian, had put to death.
• Katherine, who married Robert Grace.

Fourth, about 1532, he married Margaret Butler, widow of Richard Mór Burke and widow of Thomas FitzGerald (of Desmond), first daughter of Piers Butler, 8th Earl of Ormond. They had children:^[7]

• Barnaby (c. 1533–1581), his successor and second Baron, was invited to study in London with the boy-king Edward VI, who developed a deep affection for him.^[8] In his later years a severe rift developed between Brian and Barnaby, who was his eldest legitimate son.^[9]
• Florence, who succeeded to Upper Ossory on the death of his brother.^[10]
• Donnell, of Gortnaclea.
• Geoffrey, of Ballyawly.
• Grainne or Grizzel who was married to her first cousin, Edmund Butler, 2nd Viscount Mountgarret.

Fifth, about 1551 he married Elizabeth O’Connor. They had children:.^[7]

• Turlough (slain by Dermot O’Malloy in 1581)
• Dermot (slain by Dermot O’Malloy in 1581)

According to Carrigan^[7] Brian also fathered:

• Shane, whose mother was Joan ny Carroll.
• Teighe, of Upperwoods living 1585
• Ceallagh, of Upperwoods in rebellion 1578
• Donogh, of Upperwoods, probably Brian's second son, whose descendants took the surname Mac Fynen or Mac Kynen (modern Keenan)^[11]

According to O’Hart^[12] Brian also fathered:

• David

In 2002, researchers Ronan Fitzpatrick and Steve Zalewski published a book containing a list of all known descendants of Barnaby Fitz-Patrick, 1st Baron Upper Ossory; revised edition published in 2013.^[13][14] – it is considered the list of descendants is incomplete.^[6]

• Cokayne, George Edward (1898). Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct, or Dormant. London: George Bell & Sons.
• Carrigan, William (1905). The History and Antiquities of the Diocese of Ossory. Vol. 1. Dublin: Sealy, Bryers & Walker.
• Dunlop, Robert (1889). "Fitzpatrick, Barnaby" . In Stephen, Leslie (ed.). Dictionary of National Biography. Vol. 19. London: Smith, Elder & Co. pp. 190–191.
• Fitzpatrick, Mike (2020). "Mac Giolla Phádraig Osraí 1384-1534 AD Part II". The Journal of the Fitzpatrick Clan Society. 1: 40–71. doi:10.48151/fitzpatrickclansociety00320.
• Fitzpatrick, Ronan (24 July 2007). "Fitzpatrick Arms, Crests, Mottos and Supporters" (PDF). Fitzpatrick Clan Society (fitzsoc.com). Archived from the original (PDF) on 14 March 2012. Retrieved 13 November 2010.
• Fuller, Anne (1998). Calendar of Papal Registers Relating To Great Britain and Ireland 1492-1503 Alexander VI. Vol. 17–2. Dublin: Irish Manuscripts Commission.
• O’Hart, John (1892). Irish pedigrees: or, The origin and stem of the Irish nation. Dublin: J. Duffy.

• The Fitzpatrick Clan Society
• The Fitzpatrick – Mac Giolla Phádraig Clan Society
• Descendants of Bryan Fitzpatrick, Lord and First Baron of Upper Ossory by Ronan Fitzpatrick and Steve Zalewski
• "It is easier to forgive an enemy than to forgive a friend": Barnaby FitzPatrick and Gaelic Collaboration with the Tudor crown c.1535-1581 Archived 20 February 2020 at the Wayback Machine: By Diarmuid Wheeler (Laois Heritage Society)
• Jettisoning faith, culture and identity to serve the Crown: Elizabethan grant of land and title to Florence Fitzpatrick, 3rd baron of Upper Ossory, 1581 History Ireland