Indeed, in a society steeped in religion, celestial events could be used as propaganda.
I have copy of McCarthy's paper, and there is no mention of anything exciting happening around 774-776, nor as far as I am aware in the Irish Annals. Then again, there is no mention of a comet in AD 837, when Comet Halley made its closest approach to earth at 0.04 AU with a possible tail length of about a third of the sky (60 degrees), which has been recorded in other cultures, so making an argument that something was not observable at 775 based on Irish records is not a water tight argument. Indeed, the Annals of Ulster recorded the comet's apparition in 912 when it was 0.49 AU away from us.

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup