He was the second son of Ruairí Ó Mórdha, captain of Leix, and Margaret, daughter of Thomas Butler and granddaughter of Piers Butler, eighth earl of Ormonde. His family was one of the most important of the Irish septs.
Ruairí Caoch Ó Mórdha (fl. 1554), his father, was the son of Connell Ó Mórdha (d. 1537), and early acquired the character of a successful chieftain. On the death of Connell a dispute broke out between his three sons — Lysaght, Kedagh, and Ruairí — and their uncle Peter, who was the Tanist, in line to head the family. Peter was for the time a friend of the Butlers. Consequently, the Lord Deputy of Ireland, Lord Leonard Grey, supported the sons; and, although Peter was acknowledged chief, Grey got hold of him by a ruse, and led him about in chains for some time. Lysaght was killed; Kedagh secured the chieftainship, but died early in 1542, and Ruairí, the third brother, succeeded.
Kedagh had left a son of the same name, who long afterwards, in 1565, petitioned the privy council in Dublin to be restored to his father's inheritance. In a grant afterwards made to his eldest son his services to King Edward VI are spoken of; but an order of 15 March 1550-1 forbade any of the name of Ó Mórdha to hold land in Leix. 
At some time between 1550 and 1557 Ruairí Ó Mórdha was killed, and was succeeded by Connell Ó Mórdha, who may be the Connell Oge O'More mentioned in 1556 in the settlement of Leix . He was put to death in 1557.
In 1556 Queen Mary approved an Act "..whereby the King and Queen's Majesties, and the Heires and Successors of the Queen, be entituled to the Counties of Leix, Slewmarge, Irry, Glinmaliry, and Offaily, and for making the same Countries Shire Grounds." This shired the new counties of Queen's County (now County Laois) and King's County (now County Offaly), thereby dispossessing the rest of the O'More clan and starting the Plantations of Ireland.
Ruairí Óge and Callagh
Rory left two sons, Callagh and Ruairí Óge. Callagh, who was brought up in England, was called by the English ‘The Calough,’ and, as he describes himself as of Gray's Inn in 1568, he may be assumed to be the John Callow who entered there in 1567 (Foster, Reg. of Gray's Inn, p. 39). In 1571 Ormonde petitioned for the Calough's return, and soon afterwards he came back to Ireland, where in 1582 he was thought a sufficiently strong adherent to the English to receive a grant of land in Leix. .
Ruairí Óge Ó Mórdha, the second son, was constantly engaged in rebellion. He received a pardon on 17 February 1565-6, but in 1571 he was claimed to be dangerous, and in 1572 he was fighting the Earl of Ormond and Queen Elizabeth, being favoured by the weakness of the forces at the command of Francis Cosby, the seneschal of Queen's County, and the temporary absence of Ormond in England. The Butlers and the Fitzgeralds were united against him; but when, in November 1572, the Earl of Desmond escaped from Dublin, it was Ruairí Óge Ó Mórdha who escorted him through Kildare and protected him in Laois. He was involved in the Earl of Kildare's plans in 1574, and was taken prisoner by the English in November. But he was soon free.
The man who would order his murder, Sir Henry Sidney called O'Mordha ‘an obscure and base varlet’. When on his tour in 1575, Sidney wrote of him:
Rory Oge O'More hath the possession and settling-place in the Queen's County, whether the tenants will or no, as he occupieth what he listeth and wasteth what he will. 
Submission and war
When Sidney came into his territory, O'Mordha went to meet him in Kilkenny Cathedral (December 1575), and ‘submitted himself, repenting (as he said) his former faults, and promising hereafter to live in better sort (for worse than he hath been he cannot be)'. A new pardon was granted to him on 4 June 1576 (ib. p. 179). 
On New Year's Day 1577, a brutal massacre of the Irish gentry by Sir Henry Sidney's troops took place at Mullaghmast in County Kildare. O Mordha vowed to avenge the deaths of his countrymen. He kidnapped two men called Harrington and Cosby, relatives of important people.
Sidney invited all of the Gaelic leaders of Laois and Offaly to a peace conference at Mullaghmast. They arrived unarmed and were slaughtered with their families by the English soldiers surrounding the castle. Estimates of the dead range from 40 (the number of Gaelic lords there) to hundreds.
Ó Mórdha's wife and all but Ó Mórdha himself and one of those who were with him were killed. Ó Mórdha managed to fight his way out, and 'hacked and hewed’ Harrington so that Sidney saw Harrington's brains moving when his wounds were being dressed. Ó Mórdha rushed between a soldier's legs and escaped practically naked.
He hoped for help from Spain, and, with the backing of his friend, John Burke, he made a desperate attack on the Pale. He allied himself with some of the O'Connors, and gathered an army. On 18 March 1576-7 the seneschal of Queen's County was ordered to attack Ruairí Óge and the O'Connors with fire and sword. Sidney wrote to the council the same month: ‘Rory Oge O'More and Cormock M'Cormock O'Conor have burnt the Naas. They ranne thorough the towne lyke hagges and furies of hell, with flakes of fier fastned on poles ends’
The English had put an enormous reward for the time, £1,000, on his head, as was their practice with Gaelic lords whose land they coveted.
In an attempt to entrap Barnaby Fitzpatrick, 2nd Baron Upper Ossory, into his hands, he was killed by members of the Fitzpatrick family in June 1578, and his head was brought to Dublin Castle, which at the time was ringed by the decapitated heads of major 'rebels'.
He left a son, Eoghan mac Ruairí Ó Mórdha. John Burke, son of the Earl of Clanricarde, took charge of Eoghan. The English got hold of him after some difficulty, and he returned and recovered almost all Laois. He was killed in a skirmish near Timahoe on 17 August 1600. English propagandist Fynes Moryson called him ‘a bloody and bold young man’, Irish chroniclers the Four Masters 'an illustrious, renowned, and celebrated gentleman’. After his death the O'Mores as a sept were doomed.
Melaghlin mac Owny mac Gilla Padraigh Ó Mórdha, died 1502. | |____________________________________________________________________________________ | | | | Connell Ó Mórdha (died 1537) Pierce/Peter an Tainiste, fl. 1537. | |________________________________________________________________________________________________________ | | | | | | | | | | Lysaght (d. 1541?) Kedagh Roe (d. 1542) Ruairí Caoch Ó Mórdha, fl. 1554. Gilla Padraigh Connell Óge, d. 1557. | =Margaret Butler d. 1548. | | Kedagh/James? |____________________________ fl. 1584? | | | | Ruairí Óge, d. 1578. Calvagh/Callagh of Ballina, d. 1618. =? =Margaret Scurloug | | | |__________________________________________________ Uaithne (Owney), d. 1600. | | | | | | | | Rory O'More Lysaght Margaret dau. =Jane Barnewall (issue) (issue) (issue) (issue)
- Archbold 1895.
- Lineage and the terms of his "surrender and regrant" settlement online
- 3 & 4 Phil & Mar, c.2 (1556).
- (Cal. State Papers, Irish Ser. 1574-85, pp. 392, 412)
- cf. 12th Rep. Dep.-Keep. Publ. Rec. Ireland, p. 78.
- Carew MSS. 1575-88, f. 356
- 13th Rep. Dep.-Keep. Publ. Rec. Ireland, p. 25
- Cal. State Papers, Irish Ser. 1574-85, p. 107; cf. Carew MSS. 1575-88, f. 110.
This article incorporates text from a publication now in the public domain: Archbold, William Arthur Jobson (1895). "O'More, Rory (d.1578)". In Lee, Sidney. Dictionary of National Biography. 42. London: Smith, Elder & Co. pp. 175–176. Endnotes