The purpose of this paper is to trace the origins and decline of the diagnostic entity monomania, which became prevalent in the early 19th century and to investigate its use in Irish psychiatry.

The French psychiatric scientific writings of the early 19th century have been surveyed to identify and describe the clinical entity of monomania. The clinical description of monomania has been investigated and its cultural diffusion through literature and the arts has been reviewed. The increase in its use as a diagnosis and its ultimate decline has been documented in France, Britain and Ireland. The clinical characteristics leading to the diagnosis in Ireland have been investigated through the clinical symptoms recorded in patients accorded this diagnosis in the 19th century case books and committal documentation of the Richmond District Asylum and case books of the Central Mental Hospital.

The diagnostic entity of monomania first emerged in France in the 1820s and had disappeared from use in the hospitals of Paris by 1870. It first appeared in Ireland in the patients’ admission register of the Richmond District Asylum in 1833 and increased substantially before decreasing just as markedly with the last patient, given the diagnosis on admission being in 1878. However, the diagnosis of monomania was applied to admissions to the Central Mental Hospital as late as 1891. The Irish asylum case books have been of limited value in elucidating the clinical and symptomatological presentations leading to its use by 19th century Irish psychiatrists.

Monomania, although enjoying a scientific and cultural success in France, both within and without psychiatric circles, was a tenuous clinical entity with an ill-defined and uncertain core and fragile boundaries, both in France and more particularly in Ireland. In pure form, rarely described, its closest modern equivalent would have been delusional disorder, but case descriptions only occasionally correspond to this concept as it is understood today. Its popularity dating from around 1830 declined and by the 1870s it was in terminal decline. The factors delineating its rise and fall are unclear.