I agree that the Donner Party was no myth. I also know that there was a Patrick Breen in it.
There were, however, many Partick Breens who came out of Ireland and many Patrick Breens who lived in pioneer America. My children descend from a man named Partick Breen; and so far as I can tell, their Patrick Breen was not the man in the Donner Party, although I have heard my in-laws say that he was. My in-laws were either joking or relating a family myth.