Wikipedia (which is pretty good on literary topics):

"Contemporary critics were divided in their view of the work's authenticity, but the consensus since is that Macpherson framed the poems himself, based on old folk tales he had collected."

This is essentially what my medieval lit professor told me in college, which was not long after the Middle Ages.

Moreover, "Macpherson's fame was crowned by his burial [1796] among the literary giants in Westminster Abbey. "

Read all about it:

https://en.wikipedia.org/wiki/Ossian