MacMurrough Kavanagh, Brian (c.1530–1578), third son of Cathaoir MacMurrough Kavanagh (qv) (d. 1554) became after 1560 the principal leader of the Sliocht Diarmada Laimhdeirg and the most powerful of all the MacMurroughs. Brian first appears in the historical record on 19 August 1549 in the pardon of his father and his followers. From 1548 onwards Cathaoir mac Airt and his sons were in intermittent conflict with Richard Butler (qv) (d. 1571), 1st Viscount Mountgarret. The feud centred around persistent attempts by Butler to extend his authority over the lands of Brian's father. In March 1550 Cathaoir mac Airt seized Ferns castle from Butler, throwing the whole region into chaos. In October 1550 Sir James Croft (qv) (d. 1590) brought his forces to bear upon Cathaoir mac Airt and his sons, devastating their lands. On 4 November before a great council at Dublin, Cathaoir mac Airt acknowledged his offence and publicly renounced the title of MacMurrough. While his possessions were considerably reduced, he obtained permission to make his explanations to Edward VI (d. 1553). In addition Cathaoir mac Airt obtained a pardon dated 6 November for his kinsmen, including Brian and his brothers. Shortly before his death, Cathaoir mac Airt's prominence was further acknowledged when he was created baron of Ballyanne on 8 February 1554 – a privilege that allowed him to sit in the Irish house of lords. A second title was that of captain of his country, which ostensibly was a replacement for the abandonment of his Irish title. Cathaoir mac Airt died some time after February 1554, and he was succeeded by Murchadh MacMurrough (qv) (d. 1557), baron of Coolnaleen. The government, though, took care to balance this by making Brian's elder brother Diarmait Kavanagh (d. p.1570) tanaiste.
Diarmait and Brian in May 1556 joined the formerly loyal Fiach O'Toole (qv) of Castlekevin and Fartry (d. 1578) and Feilim O'Toole (qv) of Powerscourt and Fercullen (d. 1603) in a raid on the Pale. The raiders were driven back to Powerscourt, where they prepared the castle to resist a besieging force led by Sir George Stanley, the marshal of the army. They were persuaded to surrender when Stanley threatened to bombard the castle, and to lay down their arms. All the prisoners, including Brian and Diarmait, were brought to Dublin. There seventy-four of their men were executed publicly, but their high birth saved them and they gained their pardon on 8 June.
This close shave seems to have persuaded Brian to try to live on good terms with neighbouring English officials. Although officially at peace, Brian also developed relations with the powerful O'Byrnes of Crioch Raghnuill. At some point, probably in the early 1560s, he married Elizabeth O'Byrne (d. p.1609), the daughter of Aodh O'Byrne (qv) (d. 1579) and sister to Fiach O'Byrne (qv) (d. 1597). His marriage to Elizabeth would indicate that he had already superseded his brother Diarmait to become the most powerful of the Sliocht Diarmada Laimhdeirg. Brian was also shrewd enough to cultivate his links with the English, obtaining a grant of liberty on 26 August 1565 and later sending some of his children to an English school.
That said, towards the close of the 1560s he was being increasingly drawn into conflict with government forces. On 6 June 1566 he and several kinsmen were pardoned for unspecified offences, but the real challenge was posed by Sir Peter Carew (qv). On 21 October 1568 Carew, through an old claim of his family, petitioned the Dublin council for a grant of the barony of Idrone, Co. Carlow. This was ominous for the Kavanaghs, for a considerable portion of their lands lay in the east of Idrone. On 17 December 1568 Idrone was granted to Carew, and this was given legal effect by Lord Deputy Henry Sidney (qv) at Newcastle McKynegan in east Wicklow five days later. Any defence the Kavanaghs raised was dismissed as spurious and ill founded. The lands, in the west of Idrone, of Sir Edmund Butler (qv) of Cloghgrenan (d. 1602), the brother of Thomas Butler (qv) (d. 1614), 10th earl of Ormond, were also affected.
In June 1569 Sir Edmund rose up and was joined by the Kavanaghs, led by Brian and his son-in-law Muircheartach Óg MacMurrough (qv) (d. 1586). They linked up with the Munster revolt led by James fitz Maurice Fitzgerald (qv) (d. 1579), as well as with some of the O'Mores of Laois. Although Sir Edmund disturbed much of East Leinster, his campaign petered out after the fall of Cloghgrenan castle and the failed siege of Kilkenny. After suffering defeat at Kilmocar, Sir Edmund submitted on 1 September to Ormond and was imprisoned in Dublin castle, but the war dragged on into 1570. The resistance of Muircheartach Óg and Brian mac Cathaoir lasted until January 1570, but it was worth it as they preserved most of their lands. Indeed, Carew was now moved to compromise, to ensure the peace by dealing diplomatically with the Kavanaghs. His new approach yielded fruit, as many of them decided to hold their lands from him, as long as their tenure was not disturbed. For his activities Brian was pardoned on 25 June 1570 and was fined £20, but Sidney now sought further to stabilise the region by placating Brian. In March 1571 Brian was again pardoned, and was regranted the lands that his father had held. In return he promised to observe English law and to contribute troops to government expeditions.
In spite of this new protection, Brian was soon in trouble. Along with his brother-in-law Fiach O'Byrne, and the Furlongs of Horetown, Co. Wexford, Brian was charged with the killing of Robert Browne of Mulrankan, Co. Wexford, a prominent government supporter, on 21 April 1572. Relations between Browne and Brian's family were intimate, as his brother was married to Browne's sister. According to some sources it was Brian who killed Browne (although others say Fiach). These attacks into Wexford by Brian and Fiach were paralleled by the burnings of Rory O'More (qv) (d. 1578) in Laois during May. On hearing of Browne's death, Sir Nicholas White (qv) (d. 1593), his father-in-law and seneschal of Wexford, went to London; in early August 1572, armed with the support of Queen Elizabeth, he returned to Ireland, thirsting for revenge. The previous month Sir Francis Agard (d. 1577), seneschal of the countries of the O'Byrnes and the O'Tooles, had successfully attacked the followers of Fiach, killing his brother. Within days of his return from England, White attacked Brian and Fiach but failed to bring them to book. His assault undermined attempts being made by Gerald Fitzgerald (qv) (d. 1585), 11th earl of Kildare, and Agard to make a deal with Brian and Fiach. The latter two, it would seem, had agreed to betray the Furlongs in return for immunity from prosecution. Brian and Fiach responded to White's assault by ravaging Wexford, defeating the Wexford freeholders in August and September 1572; however, they came to peace in December 1572. As the price of their pardon, they had deserted Matthew and Robert Furlong of Horetown who were later executed. Brian's pardon, which included his wife, Elizabeth, was granted on 16 April 1573.
The turmoil caused by their disturbances seems to have further improved Brian's fortunes. Earlier, in 1566, Anthony Colclough (qv) (d. 1584) had obtained a grant of the lands of Tintern Abbey, Co. Wexford, but on 16 May 1571 he also claimed that the barony of St Mullins was included in the grant of 1566. The case was decided in Colclough's favour in August 1572. This affected Brian directly, as many of his lands were located in St Mullins. Although Brian in 1574 did not object to Colclough's victory and put in his pledges, he refused cleverly to take responsibility for any acts of his followers by refusing to give pledges for them. Lord Deputy William Fitzwilliam (qv) (d. 1599) and Sir Peter Carew were clearly concerned by the developing situation and encouraged Colclough to surrender his claim to St Mullins. This done, Brian was persuaded to hold St Mullins direct from the queen for an annual rent of £46, strengthening his hold on his lands. In spite of this, Brian remained close to the O'Byrnes of Crioch Raghnuill. With his father-in-law, Aodh O'Byrne, Brian was to parley with Ormond on 14 July 1574. He proved himself yet again adept at playing both sides, as the government entrusted him with a commission of martial law over St Mullins. However, he was instructed to use it only upon his own people and not on the settlers. The last mention of Brian was his submission on 1 October 1578 at Leighlinbridge. This remarkably able and completely ruthless man died shortly afterwards towards the end of the year, leaving at least twelve children.