The Poor Mouth (An Béal Bocht), published in 1941, is a satirical novel that stands as a noteworthy work in Irish literature. Written in the Irish language and later translated into English, the novel parodies traditional Irish peasant memoirs, offering a humorous and critical commentary on the romanticised depictions of Irish rural life and culture.

Set in a fictional, impoverished village in Ireland, The Poor Mouth tells the story of Bonaparte O'Coonassa, a character who experiences the harsh realities of rural Irish life, including perpetual rain, poverty, and the struggles of speaking the Irish language in a predominantly English-speaking country. The novel's title, The Poor Mouth, refers to the Irish tradition of downplaying one's financial well-being to avoid paying taxes or to gain charity, a practice O Brien humorously exaggerates in the narrative.

O Brien's distinctive style, characterised by its wit and linguistic playfulness, is evident throughout the novel. He skillfully satirises the tropes and styles of Irish literature and the idealized notions of Irish identity promoted by the Irish Literary Revival. The novel is not only a parody but also a critique of the sentimentalisation and politicisation of Irish culture and language.

The Poor Mouth is celebrated for its inventive storytelling, vivid imagery, and its piercing, yet humorous examination of the socio-political issues surrounding the Irish language and rural life. O Brien's work remains an important and influential piece in the canon of Irish literature, offering a unique and critical perspective on the cultural and linguistic narratives of Ireland. The novel's blend of satire, irony, and dark humor marks it as a distinctive voice in 20th-century fiction, showcasing O Brien's remarkable ability to weave complex social commentary into engaging and entertaining narratives. The Poor Mouth continues to be appreciated for its cleverness and insight, resonating with readers who are interested in Irish culture, language, and the art of satire.