MacCarthy (Mac Carthaigh), Cormac Óg Láidir (d. 1536), 9th MacCarthy lord of Muskerry, was the son of Cormac, 6th lord of Muskerry, and his wife Mary Fitzmaurice, daughter of Edmund, 9th baron of Lixnaw. Within six years of his father's death (1495), Cormac Óg had succeeded to the lordship, having removed his rivals in the traditional way. In 1498 he killed Eóghan, 7th lord, and his two sons, with the support of Thomas Fitzgerald (qv), later 12th earl of Desmond, who was married to his sister, Sheila, and became his lifelong ally. In 1501 he deposed Cormac, 8th lord. In 1510 Cormac supported the justiciar Gerald (‘Garrett Mór’) FitzGerald (qv), 8th earl of Kildare, against Toirdhealbhach Donn O'Brien (qv) (d. 1528), 22nd lord of Thomond, in a celebrated encounter in which O'Brien's bridge on the Shannon was destroyed. In 1516, when internecine warfare broke out among the FitzGerald earls of Desmond, Cormac joined the coalition of forces organised by James fitz Maurice (qv), later 11th earl (d. 1529), against Sir John, later 13th de facto earl (d. 1536). In September 1520, at the battle of Mourne, Co. Cork, Cormac, assisting Thomas fitz Thomas in his bid for the earldom of Desmond, helped to inflict a heavy defeat on James fitz Maurice, now the 11th earl. Soon after, he was presented to the lord lieutenant, Thomas Howard (qv), earl of Surrey, at Waterford by Piers Butler (qv), 8th earl of Ormond.
He captured Conor Liath O'Connor Kerry in 1524 while Conor was raiding Duhallow, Co. Cork. In December 1527 Cormac and fitz Thomas assisted Ormond in besieging James fitz Maurice at Dungarvan, following a cattle raid by James on Ormond's lands. James survived this siege, but died two years later and Cormac's ally fitz Thomas succeeded as 12th earl of Desmond. Aged about 75, fitz Thomas's chief concern was to secure the succession of his grandson and heir, James fitz Maurice Fitzgerald (qv), 13th de jure earl. He thus sought to continue the alliance with Cormac by arranging for his heir James to marry Cormac's daughter Mary. As a result of this transaction, Cormac gained control of the cantred of Kerrycurrihy, Co. Cork, for a number of years in the 1530s. When fitz Thomas died in 1534, Cormac fulfilled his obligation by providing fitz Maurice with some military support and writing to King Henry on fitz Maurice's behalf in the following year, perhaps on the occasion of fitz Maurice leaving Ireland. Cormac died in 1536, and was buried in Kilcrea, Co. Cork.
He married Catherine Barry, daughter of John, Lord Barry. They had at least five sons – Tadhg, who succeeded as 10th lord of Muskerry, Diarmaid, Eoghan, Ceallachan, and Cormac – and two daughters, Mary and Julia or Sheily, who was married three times; first to Gerald Fitzmaurice, 15th baron of Lixnaw; second to Cormac, 9th MacCarthy Reagh; and third to Edmund Butler, Lord Dunboyne.