Bunting makes no mention of the Gaelic harp using a retuning of any strings other than F and C, despite the (almost) universal historical use of the alternating B/Bb notes in Latin chant throughout the Roman church.
There is potential evidence of a diatonic harp or lyre being used to perform Latin chant in the Gaelic church: tradition associates the instrument almost as much with Gaelic saints and clergy as with druids and poets, Adam of Lennox being a likely enough 13th century example. Some of the surviving instruments have religious markings on them, eg, DO (deo oblata), and later harpers at least were hired to play in church. Carolan himself composed religious music for a liturgical context.
As the Gaelic harp cannot alternate back and forward between B/Bb within a single chant without stopping to retune, it is also worth considering the possibility that the Bb may have been disapproved of in the Gaelic liturgical chant tradition, a feature which Guido di Arezzo's writing seems to imply for the liturgical tradition of parts of western Europe during his day and a practice which Guido di Arezzo himself may have chosen to follow in later life .
The fullest information about Gaelic harp gamuts dates from after the medieval period but my research into Scottish Gaelic hexatonic modes themselves indicate the possibility that a tuning using Bb was employed for a repertoire of Scottish song using a certain set of final notes for songs and limited to a range of c-ff on the harp (cf. an seòl slinneadh). The repertoire using a smaller range of c-ff may have been transposed for lower voice to G-cc using retunable F strings.
The Bb was possibly also used by Carolan for larger ranges of Gaelic song (cf. my tables of Carolan keys elsewhere on this site) which seems to have had a different set of finals to hexatonic Gaelic song. The evidence provided by historical sources of the position of the sisters on the Gaelic harp, at middle c and the G below, may be axiomatic in helping to define the modes of Gaelic song.