Margaret Hallahan (23 January 1803 – 10 May 1868) was an English Catholic religious sister, foundress of the Dominican Congregation of St. Catherine of Siena (third order).

In 1826 she accompanied the family with which she was living to Bruges; there she tried her vocation as a lay sister in the convent of the English Augustinian nuns, but only remained there a week.

[1] She became a Dominican tertiary in 1842, and then came to England, proceeding to Coventry where she worked under William Bernard Ullathorne, afterwards Roman Catholic Bishop of Birmingham, among the factory girls.

From Coventry the community moved to Bristol, where several schools were placed under their charge, from there they went to Longton, the last of the pottery towns in Staffordshire.

At Stone a church and a hospital for incurables were built; this latter was one of Hallahan's schemes, and was begun on a small scale at Bristol.