Mac Carthaig, Diarmait Mór (d. 1185), king of Desmond, was the product of an unusual union between Cormac Mac Carthaig (qv) (d. 1138) and a lady of the declining Leinster Uí Lorcáin dynasty of Ui Muiredaig; he seems to have been the second of Cormac's three recorded sons. After Cormac was murdered in 1138 his brother Donnchad (d. 1143) ruled as king in Desmond and Diarmait may have sought refuge in Leinster. Diarmait is first mentioned in the service of Toirdelbach Ua Conchobair (qv) (d. 1156) and played a role in the defeat of Toirdelbach Ua Briain (qv) (d. 1167) in the Slieve Bloom mountains during 1141, as a result of which Toirdelbach Ua Conchobair was able to exact hostages from all of Munster, except the western portion. Diarmait succeeded as king of Desmond after Donnchad died in 1144. He became, with the exception of his father, the greatest ruler of Desmond during the twelfth century.
The early part of his reign was spent in trying to knit together the principalities of Desmond into a cohesive kingdom under the constant and heavy threat posed by the Uí Briain (O'Brien) kings of Thomond along his northern border. Early in his reign he forged a long-lasting alliance with the ruling Uí Chonchobair (O'Connor) dynasty of Connacht against their mutual enemy, Toirdelbach Ua Briain. The continual fighting between Desmond and Thomond marked the politics of Munster throughout this period. In 1152 much of Munster was devastated by the fighting between Diarmait and the Uí Briain, a struggle which Toirdelbach Ua Conchobair attempted to end by partitioning Munster between them. In 1153 Ua Conchobair, with Diarmait Mac Murchada (qv) (MacMurrough) and Mac Carthaig, invaded Thomond and banished O'Briain to Muirchertach Mac Lochlainn (qv) in Tyrone; on his return in 1154 Ua Briain ravaged Desmond.
Throughout the 1160s Diarmait was presented with continuous threats to his authority by rebellious kinsmen, perhaps as a result of his defeat in a series of encounters with the Uí Briain. The first challenge came from his cousin Donnchad, whom Diarmait killed in 1161. Another cousin, Máelsechlainn, raised a force in Kerry to fight Diarmait in 1164, but divisions within the rebel camp caused the break-up of the army and Máelsechlainn was banished to Leinster. Diarmait's regional power was proved when in 1165 Muirchertach Ua Briain submitted to him at Lismore and pledged his loyalty against his father, Toirdelbach. It is clear from the annals that Desmond was still embroiled in a bitter civil war between Diarmait and the rebels and that year Ruaidrí Ua Conchobair (qv) and Muirchertach Ua Briain brought an army into Desmond. It seems that a synod of the church was held under Diarmait's auspices at Lismore during 1166. That year the civil war in Desmond seems to have ended when Diarmait finally killed the rebel leader Máelsechlainn. Another minor revolt broke out shortly afterwards, possibly because of Diarmait's ever tightening grip, but was quickly put down. Ruaidrí Ua Conchobair confirmed the division of Munster between the Uí Briain and Diarmait in 1166 and again in 1168. In 1167 Diarmait campaigned with Ruaidrí Ua Conchobair in Tyrone and was richly rewarded. After the fall of Waterford to the Anglo–Normans, in September 1170 Diarmait defeated a large force of knights left to guard the town, a success he followed up by burning Limerick in 1171.
In October 1171, when Henry II (qv) landed in Ireland, Diarmait was threatened with a joint attack by Domnall Mór Ua Briain (qv) (d. 1194) and the Ui Mathgamna (O'Mahoneys). With that in mind he rushed to make his submission to Henry: he swore fealty, handed over hostages, and promised to pay tribute for his lands, probably more for the sake of forming an alliance with Henry against his enemies than of becoming a faithful liegeman. Relations between Diarmait and the Anglo-Normans steadily declined. They plundered his capital of Lismore in 1173, and by 1176 he was under attack from them as far westward as Muscraige. Diarmait was again forced to submit to them, and it may have been this perceived weakness that spurred his son, Cormac Liathánach, to depose him in 1176. According to Gerald of Wales (qv) and the annals, Cormac Liathánach seized his father in breach of a truce and imprisoned him, but within the year Cormac Liathánach was dead, murdered by Cathal Odhar Mac Carthaig and the nobles of Desmond with the connivance of Diarmait, who resumed his kingship.
In February 1177 the council of Oxford decided to grant the kingdom of Cork to Robert fitz Stephen (qv) and Miles de Cogan (qv). Again dynastic divisions among the Mac Carthaig dynasty increased the misfortunes of Diarmait. In November Muirchertach Mac Carthaig helped fitz Stephen and Cogan to capture the Ostman town of Cork, which resulted in widespread disturbance in Desmond. Coupled with this, Diarmait faced an opportunistic invasion of his northern border by Domnall Ua Briain, which he countered, compelling Ua Briain to come to terms. This disastrous sequence of events forced Diarmait to make substantial concessions to fitz Stephen and Cogan. Seven cantreds of Desmond were ceded to them and he agreed to pay a substantial tribute to them for the remaining twenty-four. Jeffries has suggested that Diarmait never intended to keep his word, but while he was rebuilding his military strength, he used the Cogans to defeat his enemy Donnchadh Ó Mathgamna in 1178. His attack upon the monastery of Ardfert seems to have been part of a campaign to reestablish his hegemony over the northern part of Kerry.
With the murder of Miles de Cogan and his companions at Lismore in 1182, Diarmait's pacific attitude to the Anglo-Normans changed, and he joined the great rebellion that swept through Desmond. After 1182 the Anglo-Normans regarded the agreements of 1177 as obsolete, and from then until he was murdered by Geoffrey de Cogan (qv) at a parley in 1185, Diarmait remained hostile to them. He was survived by his two remaining sons, Domnall Mór (qv) and Fíngen, who succeeded him in turn as king of Desmond. Diarmait's eldest son, Cormac Liathánach, left one recorded son, Cormac Óc, who assumed the title of king of Desmond following the imprisonment of Domnall Mór's son, Diarmait Dúna Droignéin (qv), in 1212. Upon Diarmait's release a civil war broke out between them but Cormac Óc retained the kingship, against opposition, until his death in 1244.