Colmán Bec (d. 587), son of Diarmait and prominent Uí Néill dynast, is probably to be identified as ancestor of Clann Cholmáin. His father Diarmait (qv) (d. 565) son of Cerball, king of Tara, descended from Conall Gulban (qv) son of Niall Noígiallach (qv) and possibly belonged to Cenél Conaill; his mother is named as Brea, daughter of one Colmán son of Nemand of the ruling lineage of Conmaicne. He is assigned at least two half-brothers – Áed Sláine (qv) and Colmán Már (qv) (d. 555/63), claimed respectively as ancestors of Síl nÁedo Sláine and Clann Cholmáin – and a sister, Lann, who is said to have married an Éoganacht dynast named Duí Irlúachra.
There are strong indications, however, that the persona of Colmán has been duplicated. Indeed, duplication was perhaps suspected by the scribe of the genealogical manuscript TCD MS 1298 (at 29 a 31); introducing the lineage of Colmán Bec (‘the lesser’), he adds qui prius magnus fuit (‘who originally was the great’). Certainly, it is a striking coincidence that the respective mothers of the siblings Colmán are both attached to the Conmaicne. In addition, the diminutive personal name accorded to each of the two brothers represents a curiously early adaption for secular usage of the Latin religious name Columba (‘dove’; Columbán > Colmán); it seems reasonable to view such a development in a Cenél Conaill dynastic context, whereby Diarmait gave the church-name adopted by his first cousin, the future St Colum Cille (qv) (b. 521), to a son born (most likely) in the mid 540s. Colmán Bec fits most comfortably into such a time-frame; besides, he is the only one of the brothers Colmán whose career is traceable in the sources. Though there is no mention of his wife, he had a son Cuiméne (fl. c.586), and was probably also the father of Suibne (d. 600), Fergus (d. 618) and Óengus (d. 621) – otherwise identified as sons of Colmán Már.
Colmán (Bec) emerged as a prominent figure after his father Diarmait was killed in 565, although his inclusion in Middle Irish regnal lists as king of Uisnech is probably anachronistic. Political supremacy within the dynasty, however, was assumed by Ainmere (qv) son of Sétnae, seemingly another of his father's cousins. Colmán embarked on an expedition to the Western Isles of Scotland in 567–8, and fought a battle at Ard Tommáin (apparently on Islay) in alliance with Conall son of Comgall, king of Dál Riata. This represents a significant fusion of interests: Dál Riata was closely associated with the Columban mission but, of the other dominant Ulster dynasties at this time, Dál nAraide was contesting the kingship of Tara with Uí Néill, while Dál Fiatach sought to extend its influence into northern Britain to the detriment of Dál Riata.
Colmán returned to Ireland sometime after 569, when Ainmere had been killed and the political initiative seized by Báetán (qv) son of Ninnid, whose segment of Cenél Conaill had earlier opposed Diarmait. In 573 he opposed Báetán at the battle of Femen (near Slane, Co. Meath) but was fortunate, the annalist implies, to escape with his life. Later (586) he made a bid for power when his son Cuiméne slew Báetán at Léim in Eich. The following year, however, Colmán – aged probably in his early to mid forties – was killed by Áed (qv) son of Ainmere at Belach Dathí (probably in the parish of Killucan, Co. Westmeath). In contrast to his namesake brother, Colmán Bec does feature in hagiographical tradition, although he is not accorded a prominent role. According to the Latin Life of Cainnech (qv) of Achad Bó, the capture of a nun by Colmán prompted the saint to demand his repentence. Subsequently, the saint's prayers secured the salvation of his soul (§§27, 31).
Although Colmán's sons, Suibne, Fergus, and Óengus, achieved some success in contesting dynastic supremacy with their cousins, the sons of Áed Sláine, the latter's line secured dominance as kings of Tara by the mid seventh century. Only in the early eighth century did Colmán's progeny, in the person of Murchad Midi (qv) a descendant of Suibne, re-emerge as serious contenders for the kingship of Tara. A generation or so later, the heirs of Suibne presumed to designate themselves as Clann Cholmáin, but the line descended from his brother Óengus (whose last prominent member, Follaman, died as king of Mide in 766) slid into obscurity as rulers of Caílle Follamain, a petty lordship corresponding to the baronies of Fore, Co. Meath and Co. Westmeath. Growing disparity between these parallel lines of Colmán's descendants (both of which, doubtless, wished to distance themselves from the declining fortunes of Cenél Conaill) gave rise to genealogical engineering whereby separate identities were created for Colmán, but the historical record reflected in the annals became associated with the persona of Colmán Bec.