It's not really irony. Tolkien knew what war was and didn't want anyone else to have to experience it (it's not exactly uncommon). His purpose was precisely to write a classic medieaval style myth, not an actual blood and grit medieaval story. As his interest lay in the style and language used in such works, rather than necessarily the content itself. Which I guess maybe ironically says more about our modern times focusing on the blood and grit than they would have in medieaval times. They got enough of that around them.