Clann Cholmáin Bicc were perhaps in the shadow of the neighbouring Clann Cholmáin, in full Clann Cholmáin Máir, the descendants of Colmán Bec's brother Colmán Már, whose lands lay to the south-west around the hill of Uisnech. Until the 8th century, both were in a secondary position, the leadership of the southern Uí Néill being held by the Síl nÁedo Sláine of Brega, descended from Diarmait's son Áed Sláine. During the period when the Síl nÁedo Sláine were dominant in the midlands, only one descendant of Colmán Bec was prominent, his son Óengus mac Colmáin who may, perhaps, have been King of Tara, or more probably was appointed as deputy in the midlands&mash;he is called "king of the Uí Néill" at his death—by northerner Suibne Menn.
In the 8th century, a prolonged internal struggle among the various branches of the Síl nÁedo Sláine led to their decline and the rise of Clann Cholmáin Máir and perhaps also Clann Cholmáin Bicc.
Later in the year, war is reported in the midlands. The Annals of Ulster record that Cathal—usually taken to be Cathal mac Finguine, King of Munster—was first of all defeated by Domnall Midi and Clann Cholmáin Máir Tailtiu, site of the main Uí Néill óenach, and then vuctorious against Fallomon and Clann Cholmáin Bicc at the Hill of Ward, site of the óenach of Tlachtga, second in importance only to that of Tailtiu. However, a recent reinterpretation of this record has been proposed. Rather than identifying Cathal with Cathal mac Finguine, Charles-Edwards suggests that this is Cathal mac Áeda of the Síl nÁedo Sláine, King of Lagore.
Nothing further is recorded of Fallomon until after the death of Domnall Midi in 763, at which time he appears as an ally of Domnall's kin and, in particular, Domnall's son Donnchad Midi. This alliance is presumed to have existed from the earliest part of Domnall's reign, if not before, which began when he defeated and killed Áed Allán in 743.
In 765 Fallomon is recorded as aiding Donnchad against Murchad, another of Domnall Midi's sons. Murchad was killed at Carn Fiachach, near present-day Rathconrath, fighting against Fallomon and Donnchad. Fallomon was killed in 766, at which time he is called king of Mide. The Annals of Ulster call the killing "treacherous", while the Annals of Tigernach call it "dolose" (deceitfully). Since Donnchad was the main beneficiary—he became King of Mide after this—it has been suggested that the killing was on his orders.