Nehemiah Donnellan (1649 – 25 December 1705) was an Irish lawyer and judge.
He was the son of Sir James Donnellan, Chief Justice of the Irish Common Pleas and grandson of Nehemiah Donnellan, Archbishop of Tuam. His mother was Sarah Wheeler, daughter of Jonah Wheeler, Bishop of Ossory. He graduated from Trinity College Dublin in 1666. Though he had originally intended to become a soldier, he resolved on a legal career instead, and entered Middle Temple in 1669. He was an exceptionally unruly student, who was fined for breaking down doors and for gambling at Christmas. He was called to the Irish Bar about 1672 and became Commissioner of Revenue Appeals in 1677. After the Revolution of 1688 he lived for a time in England.
He was appointed Prime Serjeant on 29 December 1692 and represented the borough of Galway in the Irish House of Commons from 1692 to 1693. In 1693 he was also made Recorder of Dublin.
He was raised to the High Court bench as a Puisne Baron of the Irish Court of Exchequer in 1695, and on 31 December 1696 was one of the Commissioners of the Great Seal of Ireland, pending the appointment of John Methuen as Lord Chancellor of Ireland in 1697. He became Chief Baron of the Exchequer on 27 December 1703, holding the office until his death. His elevation was said to be part of an effort to remove judges suspected of Tory or pro-Catholic views; but it was a little surprising that Donnellan, who was a descendant of an Old Irish, Gaelic-speaking family with Roman Catholic members, was not suspected of such sympathies himself.