Dubthach of the moccu Lugair (5th cent.?), remembered as a leading poet and brehon who flourished around the time of the introduction of Christianity into Ireland, is attached to the population group of Cenél Lugair, which is traced to one Óengus Umall of Connacht. Tradition therefore associates him with Lóegaire (qv), putatively king of Tara, and with St Patrick (qv), Fiacc (qv) who became bishop of Sletty, and Erc (qv) who became bishop of Slane. Dubthach's father is named as Lugaid. In later historical times, his population group of Cenél Lugair is featured among the forainmnecha (subject peoples) of the kingdom of Uí Enechglaiss (barony of Gorey, Co. Wexford), while Dubthach was ‘adopted’ by the Uí Chennselaig overlords of south Leinster. It is probable, however, that at the dawn of the historical period, when Dubthach presumably flourished, Cenél Lugair was represented by a dynasty of at least local importance, with a seat at Dinn Flatha Ceineóil Lugair on Tara Hill near Gorey.
Patrician hagiography places Dubthach (styled rígfhile, royal poet) in the company of his pupil Fiacc and the jurist Erc at the court of King Lóegaire on the occasion of Patrick's alleged visit to Tara (in Meath). It is claimed that Dubthach and Fiacc were the only two who rose to their feet in respect for the saint; as a result the poet and his descendants were blessed. At a later date, he allegedly met with Patrick at Domnach Mór Maige Críathair (Donaghmore, barony of Gorey), where he was party to the choosing of Fiacc as bishop.
Dubthach was traditionally associated with the compilation of the corpus of laws known as ‘Senchas Már’, the prologue to which maintains that this task was carried out at the instance of Patrick. The assignment of the ‘Senchas’ to the early date of 438 in the Annals of Ulster arises out of later efforts to synchronise Patrick, and therefore the ‘Christianisation’ of the laws, with the mission of Palladius (qv) to Ireland. Dubthach's role as revisor is amplified within the corpus, especially in the tract ‘Córus Béscnai’, which probably dates to the early eighth century. It is related that, arising out of a judgement Dubthach had to give against Lóegaire for the slaying of Patrick's charioteer, a major revision of the laws was planned to bring them into line with Christian values. Dubthach is also brought into contact with Brigit (qv) of Kildare; according to ‘Bethu Brigte’ (§14), he was refused the hand of the future saint in marriage, but she helped him to find the bride intended for him.
A selection of poetry of later date is ascribed to Dubthach. A poem that carries his name in ‘Lebor na Cert’ (The Book of Rights) decrees that no province owes hospitality to a poet who does not know its appropriate rents and stipends. Three poems in the Book of Leinster are also assigned to Dubthach; these celebrate the victories of the proto-historical Leinster king Énnae Cennselach (qv) and his son Crimthann. One of the verses claims that Dubthach built an ecclesiastical foundation with an oratory and a stone cross. It appears that certain ecclesiastical interests, especially in parts of Leinster, later identified with Dubthach or at least with Cenél Lugair. A site called Cellín U Lugair (apparently situated in the barony of Talbotstown Lower, Co. Wicklow) is included among the twelfth-century properties of Glendalough. In the genealogies, Dubthach is named as the father of saintly church founders. His numerous sons are said to have included Fachtna of Cell Tómma (Kiltoom, barony of Fore, Co. Westmeath) and three others who, with a certain Mochommóc of the moccu Lugair, had a church at the aforementioned royal site of Dinn Flatha – where, it may be noted, the cult of St Conláed (qv) was also commemorated. A grandaughter of Dubthach, a certain Deidiu daughter of Trian, was allegedly the mother of several saints including Daig (qv) son of Cairell, and Diarmait (qv) of Inis Clothrann.