In the story, the unnamed narrator describes the final fate of his good friend, Denys Barry, an Irish-American who reclaims an ancestral estate in Kilderry, a fictional village in Ireland.

Barry ignores pleas from the superstitious local peasantry not to drain the nearby bog, with unfortunate supernatural consequences.

The story was written at speed and to order, for oral delivery as an after-supper "shocker" for a Hub Club gathering of amateur journalists in Boston on March 10, 1921.

There is also a further autobiographical element in "The Moon-Bog" – Lovecraft had seen his boyhood haunt of Cat Swamp purchased by the city authorities with the declared aim of protecting it from developers.

[4] In his story Lovecraft leans on authentic Irish legend, that of the first invaders of Ireland, the Partholonians – and the sudden plague that wiped them out in around 1200 BC.