Erm, the Táin and other Irish/Gaelic/Goidilic epics would generally come from the Iron Age. The surviving version of the Táin is the (rather biased) version taken down by monks in the early middle ages of a version of the legend from the 400s to 500s. The legend is older than that. (Trust me, I have an exam on this in Irish on Friday)
Regarding epics and historical value, the Iliad is an exaggerated version of the state of affairs at the time. I.e. Chariots, single combat, etc, in a highly stylised form.
The Táin is not unbelievable for the military detail in it, rather the scale of that, as well as the prowess of those involved is exaggerated.