Cellach Cualann (d. 715), son of Gerthide and overking of Leinster, was the last and arguably the most colourful representative of the Uí Máil dynasty to hold that dignity. His sobriquet associates him with Cualu, the Bray area in north Wicklow. While Cellach's father Gerthide and his grandfather Dícuill may have been local rulers, neither achieved provincial kingship. His closest relative to reign as overking of Leinster was his father's first cousin, Fiannamail (qv), while his immediate predecessor was apparently Bran Mút (d. 693) of the rival dynasty of Uí Dúnlainge. Apparently, Cellach had at least one brother, Fiannamail, who died (736) as abbot of Clonard.
From an early stage in his career, Cellach appears to have followed a policy of consolidation within Leinster and of alliances with the ruling lineages of Brega. According to the ‘Banshenchas’ (lore of women), he personally contracted four marriages, which displays a pattern of political alliances paralleled and enlarged upon by the dynastic unions involving his own daughters. One of his wives was Caintigern daughter of Conaing who, if her surviving pedigree is correct, was his own first cousin. The others were Mugain, who belonged to the Leinster dynasty of Uí Bairrche, Muirgel daughter of Muiredach of the Cianachta (probably of Meath rather than Glinne Gaimen), and Bébail (d. 741) daughter of the king of Tara, Sechnassach son of Blathmac (qv) (d. 665), who belonged to Síl nÁedo Sláine, an Uí Néill dynasty. He had at least five sons: Fiachra, Fiannamail, Áed, Crimthann, and Etarscél.
Of his five daughters, Coinchenn (d. 743) was bestowed on the Uí Dúnlainge dynast Murchad (qv) son of Bran Mút, presumably in an attempt to reach a modus vivendi with the most powerful rivals of Uí Máil within northern Leinster. Two others were married into the Síl nÁedo Sláine to preserve the alliance with Uí Néill: Derbfhorghaill and Muirenn (d. 748) were respectively married to the king of Tara Fínshnechtae Fledach (qv) and to his cousin Írgalach (qv) son or grandson of Conaing, king of Brega. Muirenn, according to one version of the ‘Banshenchas’, was also wife of the Cenél Conaill king of Tara, Loingsech (qv) son of Óengus, although this appears to be a scribal error. Other than her name and date of death (731), nothing is known of his daughter Coblaith.
A fifth daughter, Caintigern (qv), whose later career led to her being commemorated as a saint, was apparently married to a Dál Riata dynast named ‘Feriacus’. The latter is probably to be identified with Feradach grandson of Artúr who, along with Cellach, features as a witness to the promulgation (c.697) of the Law of Adomnán (qv). Despite his subscribing to this ecclesiastical law promoted by the head of the Columban federation, Cellach later became, at least in death, persona non grata with the Columban community. He features in the ‘Leinster episode’ of the tenth-century ‘Betha Adomnáin’ (Life of Adomnán) in a decidedly unfavourable light, along with his alleged protégé Dubgualai, abbot of Glendalough. The dynasty of Uí Máil was closely associated with Glendalough, the interests of which, in the eighth century, were in conflict with those of Kildare. The latter foundation already had a history of close involvement with the rival dynasty of Uí Dúnlainge, in turn allied to Clann Cholmáin which sought to contest the kingship of Uisnech with Síl nÁedo Sláine, allies of Cellach. In the course of the eighth century Clann Cholmáin became closely identified with Columban interests. In 704 Cellach blocked an attempted invasion of Leinster at Clane, Co. Kildare, where Clann Cholmáin dynast Bodbcath Midi, a brother of Murchad Midi (qv), was slain and his ally Fogartach (qv), who belonged to an out-of-favour Síl nÁedo Sláine line, was put to flight. An incursion into Mide, which was reversed at Garbshalach with the loss of the king of Uí Fhailge was perhaps intended as a retaliation on the part of Cellach.
Though described as rí diabul-Laigen (king of double-Leinster) in a gloss (not necessarily contemporary) in the text of Cáin Adomnáin (the Law of Adomnán), Cellach's authority over the south of the province certainly did not go unchallenged. A battle (709) at Selgg (near Glendalough) probably achieved the objective of curtailing the ambitions of the Uí Chennselaig dynasty, but still cost the lives of two of his sons, Fiachra and Fiannamail, along with a contingent of British mercenaries. These soldiers, or at least their leaders, were probably displaced noblemen from Rheged, a north-British kingdom which collapsed towards the end of the seventh century. Their activities in Ireland can be traced in a progression from north to south over a span of years. Presumably, Cellach initially engaged them through his connections with Dál Riata and Síl nÁedo Sláine. In 702 a group of Britons had slain his son-in-law, Írgalach of Brega, on Ireland's Eye. Whether this killing represented a straightforward dispute between the parties concerned, or involved complicity on Cellach's part, is not clear.
After the death of Cellach (715), it seems that his son-in-law Murchad son of Bran Mút was able to claim the overkingship of Leinster at the expense of Cellach's sons. The fortunes of Uí Máil thereafter declined as Cellach's sons were slain in turn, defending the interests of their dynasty against Uí Dúnlainge and its allies: Áed in 719, Crimthann in 726, and Etarscél the following year. Two grandsons were slain in 744, but the dynasty still contrived to retain control of Cualu for a further period. Another grandson, Tuathal son of Crimthann, who died in 778, was the last of the line to be styled rí Cualann. Later descendants of Cellach ruled a small territory in south Co. Dublin and were described as kings of Uí Chellaig Cualann.