It's hard to focus when you have a deadline looming. Believe me, I know!
I think the point you're making is that stories about a hero have to change when a culture's circumstances change. If a country is invaded by a seafaring enemy, their heroes have to become expert sailors who can vanquish any attack from the sea.
You say you are focusing on religion, language and mythology. The mythology part is probably the least complicated, since it refers to a fixed set of stories. Do you notice an changes in the stories over time? Do the heroes change in any major ways (acquire new abilities, compromise their morals, develop new strategies for coping with an enemy, etc.)?
Changes in a country's religious beliefs (as when Christianity came to Ireland) would be bound to have an effect on stories about heroes. A hero in the Christian era would probably have an entirely different outlook from earlier, pre-Christian heroes.
As to language, that always changes over time. Is new vocabulary necessary to describe changing circumstances in a culture? Does that affect the way the hero-stories sound? You probably have a much better grasp than I do on the points you want to make about language.
Good luck! Breathe deeply, and listen for the muse whispering in your ear (that's what I always tell myself when I get stuck for an idea!).