OG Benén, St Benignus
There are a number of saints in Ireland going by the same name as the one commemorated in Cill Bhanain. There is one prominent saint, however, and that is probably who is represented here.  He was reputed to be a companion of St Patrick. In the earliest literature relating to Patrick, he goes under the Latin name of Benignus (“good, friendly”), and indeed, the name seems to be a Latin one in origin, Gaelicised initially as Benén, later Beanán, Banan. In a list of St Patrick’s successors in Armagh, he is recorded first, the implication being that he was St Patrick’s immediate successor. In late 7th-century texts from Ireland relating to Patrick, Benén is portrayed as one of his young clerics who undergoes a trial by fire in a competition with druids.  The earliest account of him, in the long text by Bishop Tírechán on St Patrick’s journeys around Ireland, describes how he was the son of a man whom Patrick baptised, who pleaded with Patrick to take him with him when he left:
“When in the morning they got up and Patrick, having blessed the father of Benignus, was about to mount his chariot, with one foot in the chariot and the other on the ground, Benignus held on to Patrick's foot with his outstretched hands and exclaimed: 'Allow me to be with Patrick, my real father', and Patrick said: 'Baptize him and lift him up into my chariot, for he is the heir of my kingdom.' This is bishop Benignus, Patrick's successor in the church of Armagh." (Bieler 1979) .
The name Cill Bhanain is matched by that of Kilbennan in Co. Galway in Ireland, where there is a longstanding tradition of its being dedicated to Benignus, the companion of St Patrick (this goes back to the 7th-century work of Tírechán, mentioned above). The pronunciation of the name (recorded in 1978 as [k^il’vanan, k^il’banən], see logainm.ie) seems to reflect this, as also the form recorded in 1838, Kilbannon. It is recorded in 1114 (Annals of Tigernach 1114.2) as Cell Benéoin and we might imagine this was roughly the form of the South Uist name when it was first coined.
While a dedication to a saint associated with Patrick, and with Armagh, might make us wonder about both of these as contexts for his cult in South Uist, it is worth considering that to Benén was attributed a long poem on the conversion of the Scandinavians of Dublin, in which their rights and dues were spelled out (Dumville 1993, 258-64) . Thus, rather than considering this saint from the point of view of an early church dedication, we might wish to pair it with the dedication to St Olaf, as a commemoration closely related to the Scandinavian world of the Irish Sea and the North Atlantic.
On the other hand, a saint, likely to be this one, is commemorated in two places in Scotland. First, at Kildavanan in Bute, the saint has an honorifc Do ‘Your’ in front: Do Bhanain “your Benén”. (see Márkus 2012, 391-95; but also Butter 2007, 55) . The other potential dedication is at a site now called Clashbenny, but which appears in the 13th century as Ecclesdouenauin (1214--again with Do prefixed) and Egclisbanyn (1253). This belongs to a series of place-names which look like they are of fairly early coining, perhaps the period around 700 (see Barrow 1983; Clancy 2014) . This suggests that in eastern Scotland, at least, there was a cult of Benén in the eighth century. These two other Scottish commemorations suggest that there is no reason that Cill Bhanain on South Uist couldn’t be an earlier church dedication. What primary association he might have had in Scotland is uncertain--at a guess perhaps this was an association with St Patrick, but there is no other evidence to support this (like nearby dedications to Patrick himself). We should, of course, leave open the possibilities that, first, these are not all dedications to the same saint; and second, that the Banan commemorated in South Uist is not the same one as Patrick’s companion.