And then, there is the second half of the story (in some versions): After Oisín got back to Ireland and before he died, he met with Saint Patrick, and the two old men developed a deep respect for each other, even though Oisín refused to convert to Christianity.

In an earlier version of the story, dating from the 12th century, the meeting was between Patrick and Oisín's companion Cailte, and ends with Cailte embracing the new faith...

W. B. Yeats wrote a long poem about the Oisín version of the tale, btw...