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John Casey (born 12 May 1820 at Coolattin, Kilbehenny, County Limerick, Ireland, died 3 January 1891 at Dublin) was a respectedIrish geometer. He is most famous for Casey's theorem on a circle that is tangent to four other circles, an extension of the problem of Apollonius. However, he contributed several novel proofs and perspectives on Euclidean geometry. He and Émile Lemoine are considered to be the co-founders of the modern geometry of the circle and the triangle. He was educated locally at Mitchelstown, before becoming a teacher under the Board of National Education. He later became headmaster of the Central Model Schools in Kilkenny City. He subsequently entered Trinity College as a student in 1858, and was awarded the degree of BA in 1862. After a distinguished career in Trinity he was elected to the Royal Irish Academy and in 1880 became a member of its council. In 1869, Casey was awarded the Honorary Degree of Doctor of Laws by Dublin University. He became a Professor of Higher Mathematics and Mathematical Physics at the newly founded Catholic University. He left this post in 1881 and the Royal Irish Academy conferred upon him the much coveted Cunningham Gold Medal. His work was also acknowledged by the Norwegian Government, among others. He was elected a member of the Societe Mathematique de France in 1884 and received the honorary degree of Doctor of Laws from the Royal University of Ireland in 1885.

Major works

* On Cubic Transformations (Dublin, 1880)
* A Sequel to the First Six Books of the Elements of Euclid (Dublin, 1881)
* The First Six Books of the Elements of Euclid (Dublin, 1882)
* A Treatise on the Analytic Geometry of the Point, Line, Circle and Conic Sections (Dublin, 1885)
* A Treatise on Elementary Trigonometry (Dublin, 1886)
* A Treatise on Plane Trigonometry containing an account of the Hyperbolic Functions (Dublin, 1888)
* A Treatise on Spherical Geometry (Dublin, 1889).