There isn't a single, universally accepted Gaelic spelling for "Brendan." The spelling depends on which Gaelic language you mean (Scottish Gaelic or Irish Gaelic) and the specific orthographic conventions used. Both languages have their own nuances.
* Irish Gaelic: The most common spelling is Breandán. You might also see variations like Braindeán.
* Scottish Gaelic: The most common spelling is Breanndan.
Therefore, to be precise, you need to specify whether you mean Irish or Scottish Gaelic.