Diarmait (Diarmait mac Maíl (Máel) na mBó) (d.1072), Gaelic lord, was the son of Donnchad son of Diarmait, otherwise known as Donnchad Máel na mBó (qv) who died in 1006 when Diarmait was a young boy. His mother is named as Aífe daughter of Gilla Pátraic in the Banshenchas (women-lore); according to the same source he had a daughter of the same name. More is known about his son, Murchad (qv), whose mother was Derbforgaill, daughter of Donnchad (qv), son of Brian Bórama (qv). He predeceased his father by two years. Called tigerna Gall (lord of the foreigners [the Dublin Norse]) in the Annals of the Four Masters in 1059, he appears to have been made governor of Dublin by his father before that date. ‘An active and able lieutenant’ for Diarmait, in the words of Donnchadh Ó Corráin, Murchad may have managed affairs in Leinster, while his father was pursuing expansionist activities elsewhere (Ó Corráin, ‘Career’, Part II, 18–19).
It was some time, however, before Diarmait could consider expansion and his early activities suggest that he was concerned with enforcing his authority close to home. In 1032 when he is first mentioned in annalistic sources and again four years later, he dealt with potential rivals, killing Tadc Ua Guaire and blinding a former king of Uí Chennselaig, Ruaidrí son of Tadc. In 1037 he plundered Waterford with some success. His constant ally in this period was Donnchad (qv) son of Gilla Pátraic of Osraige; his chief rivals were Uí Dúnlainge, whose ruler Murchad (qv) son of Dúnlaing (qv) is accorded the title ‘king of Leinster’ in king-lists. With the support of the Munster leader, Donnchad son of Brian Bórama, Murchad attacked Diarmait's stronghold of Ferns in 1041; he had already slain the latter's brother, Domnall Remur (qv), earlier that year. Murchad himself died a year later and Diarmait slowly consolidated his power.
Relations with his southern counterpart and father-in-law, Donnchad son of Brian Bórama, fluctuated. His marriage with the Munsterman's daughter Derbforgaill reflects a cordial relationship at an early stage in his career; this was replaced by one of strife as Diarmait became increasingly powerful. He was forced to give hostages to Donnchad in 1048; yet this in no way entailed submissiveness on his part since he soon attacked the Déisi and two years later made an incursion into Mide. Donnchad was again forced to intervene, once more taking the hostages of Leinster and those of Osraige. In 1052, however, Diarmait got the upper hand, plundering Fine Gall and forcing its Norse ruler, Echmarcach (qv) son of Ragnall, an ally of Donnchad, to flee overseas. In a master stroke he made himself king of Dublin, thus acquiring a key strategic base north of his own territorial heartland. Moreover, his control over the settlement appears to have been real. In the first place, Donnchad son of Brian was quick to attack Dublin the following year, ‘hitting at this new weapon in Leinster's arsenal’, in Seán Duffy's words (‘Irishmen’, 97). In addition, as Duffy further notes, the Dublin Norse were part of Diarmait's contingent when he raided Munster in 1054 and again in 1057; indeed on the latter occasion they are described as gaill m. Muíl na mBó (the foreigners of the son of Máel na mBó). Sometime in these years too Harold and Leofwine, sons of Godwin, earl of Wessex, received hospitality at Diarmait's house, perhaps at his Dublin court, as Harold's sons did some years later, seeking refuge with him after the battle of Hastings in 1066 (Flanagan, Irish Society, 58). As ruler of Dublin and Leinster, Diarmait's power was also acknowledged across the Irish Sea.
Further south his supremacy was resisted, Donnchad son of Brian Bórama refusing to succumb. In the latter's troublesome nephew, Tairdelbach Ua Briain (qv) (1009–86), Diarmait, however, found a protégé whom he assiduously cultivated in an effort to bring down the Munster king. They first appear together raiding in Munster in 1054 where they have the might of Connacht and Osraige also in tow; unsurprisingly, Donnchad is defeated, his fort at Dún Trí Liac being raided in the process. A two-pronged approach was adopted in the following years: while Tairdelbach attacked Donnchad's home base of Clare, Diarmait, with his Norse allies, plundered Scattery Island in 1057. A year later, the powerful allies raided as far as Limerick, Donnchad's followers burning the settlement before them ar omun ind lochta aile dia loscud (‘lest the other party should burn it’) (A.I. 1058). This defiant gesture was of no avail, Donnchad being heavily defeated shortly afterwards in the battle of Sliab Crot. In an effort to withstand Diarmait, in 1059 the much weakened Munster ruler submitted to the Connacht king, Áed in Gaí Bernaig Ua Conchobair (qv). One year later, he finally accepted the inevitable, acknowledging Diarmait's supremacy by accepting gifts from him. The Leinster king was unrelenting: along with his Munster subordinate, Tairdelbach, he attacked Donnchad again in 1062, Diarmait's candidate finally assuming the kingship of Munster the following year.
Diarmait and Tairdelbach continued to campaign together, going on a successful hosting against the king of Connacht in 1067. The Munster ruler demonstrated his allegiance to his Leinster ally the following year, receiving the sword of his grandfather, Brian Bórama, and the standard of Edward the Confessor from Diarmait. In 1070 he submitted to him again and one year later Diarmait accompanied Tairdelbach to Munster co farcaib bennachtain ic feraib Muman (‘and he bestowed a blessing on the Munstermen’) (AU 1071). Diarmait may well have sought assistance from his southern allies on this occasion since a branch of his own family had done battle with him earlier that year. Whether it was forthcoming or not is another matter; it is noteworthy that Tairdelbach was fighting alongside the enemies of the Leinster king in the battle in Mide in which Diarmait was slain on 7 February, 1072.
Notwithstanding the fact that Diarmait's supremacy was being challenged somewhat in the last year or so of his life, for almost twenty years he had been the most powerful player on the Irish political stage. An astute operator, he made skilful alliances which he used to further his own career. In Tairdelbach Ua Brian he may have met his match, as is evident from the manner in which the Munster king ravaged his protector's kingdom immediately after Diarmait's death. Tairdelbach's uncle, however, was powerless to withstand Diarmait's relentless and well-placed military assaults. Supreme in Ireland's southern half and with a firm grip on Dublin, this strategic ruler did in fact deserve the title rí Érenn co fresabra (king of Ireland with opposition) accorded him in the Book of Leinster, since he was indeed the most powerful man in Ireland in his time. An outward-looking leader, he may not have been ri Breatan ocus Indsi Gall (‘king of Britain and of the Hebrides’), as the obituary of him in the Annals of Tigernach somewhat hopefully claims. Utterly realistic, however, is its description of him as ri . . . Atha Cliath ocus Leithi Mogha Nuadhad (‘the king of Dublin and of the southern part of Ireland’), which territories Diarmait ruled with an iron grip.