The Sea Bell poem is Not autobiographical for JRR Tolkien; but The Death of St Brendan ("Imram") is

The Voyage of St Brandon - by Edward Reginald Frampton

Over at the Notion Club Papers blog, I argue that The Sea Bell poem is Not autobiographical for JRR Tolkien but instead for Frodo in-universe; however, The Death of St Brendan ("Imram") poem is expressive of Tolkien's own experience and convictions.