Alfred Smyth (Celtic Leinster) provides an interesting theory on the rise of the Úí Cheinnselaig. He cites an early center of power of Úí Cheinnselaig based at Rathvilly in Carlow, as shown by early tradition in the Vita Tripartita of St. Patrick, as well as by earlier documents in the Book of Armagh. The monastery of St. Mullins in southern Carlow had earlier claims to Úí Cheinnselaig patronage than did the house of Ferns in Wexford, which by the 11th century became the overall center of their dynastic power. He speculates the Úí Cheinnselaig 'invaded', not earlier than the 5th century, through the pass of Gowran from Ossory, in order to explain how St. Mullins had earlier connections to the group, as well as to explain how the Úí Cheinnselaig broke the power of Úí Bairrche by seizing the Slaney valley from Rathvilly to Tullow, thereby separating the Úí Bairrche of north Carlow from those of southern Wexford.

In addition, the Foathirt, allies of Úí Bairrche, shared the same fate and were separated into two groups. Smyth goes on to speculate the Úí Cheinnselaig expanded into Wexford down the Slaney valley through a pass between the Balckstairs and the Wicklow Mountains.