Tomás Ó Criomhthain (anglicised as Tomas O'Crohan or Thomas O'Crohan; 21. December 1856–1937) was a native of the Irish-speaking Great Blasket Island 3 kilometres (1.9 mi) off the coast of the Dingle Peninsula in Ireland. He wrote two books, Allagar na h-Inise (Island Cross-Talk) written over the period 1918–23 and published in 1928, and (The Islandman), completed in 1923 and published in 1929. Both have been translated into English. The 2012 translation by Garry Bannister and David Sowby is to date the only unabridged version available in English (earlier versions were redacted being considered too earthy).
His books are considered classics of Irish-language literature containing portrayals of a unique way of life, now extinct, of great human, literary, linguistic, and anthropological interest. His writing is vivid, absorbing and delightful, full of incident and balance, fine observation and good sense, elegance and restraint.
He began to write down his experiences in diary-letters in the years after World War I following persistent encouragement by Brian Ó Ceallaigh from Killarney. Ó Ceallaigh overcame Ó Criomhthain's initial reluctance by showing him works by Maxim Gorky and Pierre Loti, books describing the lives of peasants and fishermen, to prove to Ó Criomhthain the interest and value of such a project. Once persuaded, Ó Criomhthain sent Ó Ceallaigh a series of daily letters for five years – a diary – which the latter forwarded to scholar and writer Pádraig "An Seabhac" Ó Siochfhradha for editing for publication. Ó Ceallaigh then convinced Ó Criomhthain to write his life story and best-known work, An t-Oileánach.
Ó Criomhthain had four sisters, Maura, Kate, Eileen and Nora, and a brother, Pats. His personality was well-grounded in intimate affection and respect for his parents who lived to a ripe old age. The only, minor, disharmony arose between himself and Nora who was five years older than he. She had been the family favourite until Tomás arrived "unexpectedly". Her jealousy of him created friction between them.
He married Máire Ní Chatháin in 1878. She bore ten children but many died before reaching adulthood: One boy fell from a cliff while hunting for a fledgling gull to keep as a pet among the chickens; others died of measles and whooping cough; their son Domhnall drowned while attempting to save a woman from the sea; others were taken by other misfortunes. Máire herself died while still relatively young. Their son Seán also wrote a book, Lá dar Saol (A Day in Our Life), describing the emigration of the remaining islanders to the mainland and America when the Great Blasket was finally abandoned in the 1940s and 50s.