Yeah, I must say within D&D I doubt it's easy to bring it back to Celtic context. Then of course Druids with their nature worship from the very get go were some weird hippy cult in D&D, not original augurs and tribal memory keepers. Sure they were associated with oaks - but most of priesthood in Northern Indoeuropean culture cared about oaks as those were holy trees (that's why Saint Boniface cut one when preaching among Saxons). Going from there to nature-tree magic was already quite off.
And augury does not works that well for RPG for obvious reasons.
So I'd probably go with what's already there - D&D as it's own genre, with own archetypes. Which means bard is somehow musical quasi-magician.
I sort of enjoy them being truly jacks of all trades. It would sort of pull together their insane history through edition - druidic, divine, rougish, magical magic boosters, arcane lore, arcane leader in 4e, psychic spellcaster in PF2. Put it all into big mixer and crush together.
I'd go more with sage as basic social role, with minstrel being just on many options of jacky nature of bard, but their overall combat-utlity roles I'd well how to say it. If we were using 4E, he'd be able to take powers from each power source and class (though with power limits stronger than them) and be forced to take them from at least 3 sources. Mess.