John Casey (12 May 1820, Kilbehenny, County Limerick, Ireland – 3 January 1891, Dublin) was a respected Irish geometer.

He and Émile Lemoine are considered to be the co-founders of the modern geometry of the circle and the triangle.

He subsequently entered Trinity College Dublin in 1858, where he was elected a Scholar in 1861 and was awarded the degree of BA in 1862.

[2] In 1869, the University of Dublin awarded Casey the Honorary Degree of Doctor of Laws.

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.