He erected various oratories near the rivers Cleddau, Gwaun, and Caman and at the foot of Mynydd Carningli (translated as 'Mountain of the Angels'), which was his most famous foundation.

The descriptions of his adventures (including amorous and ghostly encounters) display a degree of humour unusual in the writers of saintly lives.

However, Egerton Phillimore rejects this identification A 'Brennach Wyddel o'r Gogledd' or Brennach the Irishman of the North [of Britain] appears in the Welsh Triads: Rachel Bromwich does not believe Saint Brynach is meant.

Dedications of churches to Brynach in Pembrokeshire include Nevern, Dinas Cross, Llanfyrnach, Henry's Moat and Pontfaen.

The distribution of these is similar to that of Ogham stones in south Wales, and defines a distinct Irish-influenced province that existed in the Age of the Saints.