Like Zetsumei said,
there's been a fairly heavy implication that the "Irish heroes", as we would know them and as they're all being sourced for the Mhach raids and the upper echelons of the voidsent in general, were once heroic figures more like their real-life legends in the process of trying to stop the Rejoining on their world. The problem is, as the lore book states, their use of auracite changed them; by the time they had "won", they were no longer recognizable as the heroes they had been or we would know.
I've been wondering if we might get a glimpse of the heroes as they were before their decay, or if even one of the old heroes somehow escaped the fate of his fellows and is fighting a doomed war to stop the others (Diarmuid Ua Duibhne, maybe? Or Fionn Mac Cumhaill, or Fergus Mac Roich?), but that's honestly just potholing. They could go there, but it also feels like, for now at least, the Voidsent "story" is wrapped. There's a decent chance we'll run into some of them again, or run into new ones, but what happened is clear and the immediate threat from the Thirteenth has been dealt with.
For a little more background, a lot of the above spoiler stuff is the result of the legacy of previous Final Fantasy games - it all got started with "Cuchulainn" (or Queklain for you PS1 vets) being one of the Lucavi in Final Fantasy Tactics, and there at least it seems like the name was nearly drawn out of a hat. The Lucavi demon of that game has essentially nothing to do with the various Ulster Cycle myths of Setanta Cu Chulainn, and none of the other Lucavi reference any cycle of Irish myth (they're largely drawn from Judeo-Christian apocrypha). What's going on in XIV seems to be an attempt to take that seemingly out-of-place character and build a wider mythology around its current state, leading to the spoilered situation above.
I would imagine this also seems more curious these days because the heroes of Irish-Celtic mythology have been getting quite a bit more exposure in recent years both in the Anglosphere and in Japan, especially in Fate/Grand Order, the hugely successful mobile game that's become the new flagship product of the Fate franchise. Pretty much all of the Ulster Cycle characters in XIV to date (save Ferdiad, funnily enough) have appeared in F/GO, and they tend to be a lot truer-to-the-text due to Fate's premise of summoning "Heroic Spirits" to assist with various escapades.
I suspect this actually informed XIV's Scathach to some degree - F/GO's Scathach has become virtually iconic (it's been publicly acknowledged she's pushed more gacha purchases than any other F/GO character) and the various XIV design teams probably wanted to make sure there was some distance between their Scathach and the one who has risen to meteoric popularity lately (and, in fact, literally alongside Heavensward's run). Thus, the design of "our" Scathach being somewhere between Kingdom Hearts and Tim Burton, as opposed to the mysterious warrior witch of legend.