I don't know if it would be used at such an early period, but in later times the phonetic shortening "Abú!" was a major element of many Gaelic war-cries. Pronounced "ABOO" or "AVOO" (depending on accent and so on) and shouted very loud and drawn out. I think it's a contraction of "a bhuaidh" which is "victorious". Used in the form "X Abú", inserting the name of leader/king or of a clan, or sometimes a symbol. eg. the war cry of the Ui Neill of Ulster was "an lamh dearg abú" meaning "the Red Hand victorious"
As I said that's mostly described quite a bit later (like 14th-17th centuries) but it's worth knowing. I also read something about Brythonic/Welsh warcries not to long ago, I'll have a look for it.