Donnellan, Nehemiah (1649–1705), MP and chief baron of the exchequer, was born in Dublin, third son of Sir James Donnellan (qv), MP and lord chief justice of the common pleas, and his second wife, Sarah, fourth daughter of Jonas Wheeler (1543?–1640), bishop of Ossory, and widow of Matthew Tyrrell of Dublin. After being educated in Dublin under a Mr Shaw, Donnellan entered TCD in February 1665/6 and matriculated in 1666. Ireland having at that time no institution for legal qualification, he was admitted (1669) to Middle Temple, London. While there, he was fined ‘for breaking open the doors of the hall, parliament chamber, and kitchen at Christmas, and setting up a gaming Christmas, persisting after Mr Treasurer's admonition, and continuing the disorders until a week after Twelfth day’ (Middle Temple records, iii, 1253). On completing his studies, he returned to Ireland in 1677 and was appointed a commissioner of appeals in revenue cases.
The Donnellans were a family of ancient Gaelic stock in Galway, whose fortunes reflected the complexities of seventeenth-century politics. Baron Donnellan was the grandson of Nehemias Donellan (qv) of Ballydonelan, Galway, protestant archbishop of Tuam (1595–1609), several of whose descendents later fought for James II (qv). Baron Donnellan's father James held judicial office before, during, and after the interregnum. During the Jacobite war of 1689–91, Donnellan himself was attainted by King James's parliament and fled to England with his children (his first wife having died in 1684) and his mother. While there, he was called to the English bar in 1689 by the Middle Temple, which at the same time voted to contribute to the relief of Irish protestants. Following the defeat of the Jacobites in 1690, he returned to Ireland and was appointed king's counsel. The war, Williamite confiscations, and subsequent regrants often put his interests at odds with those of the Galway Donnellans.
If Donnellan had suffered for his earlier loyalties, his judicial career owed much to his political flexibility in office. A member of the Connacht bar, he subsequently became recorder (magistrate) of Galway (1691–4) and Dublin (1693–5) as well as MP for the town of Galway (1692–3) in the brief parliament of William III (qv). He was prime serjeant (1693–5), resigning that position when raised to the bench (1695) as third baron of the exchequer, partially the result of faith in his whig principles. Donnellan was appointed a commissioner of the great seal (1697) and was awarded a quarter of lands in Galway and Roscommon that he claimed as a discoverer on behalf of the crown (1697). In 1700, he was briefly considered for the English bench and was eventually made chief baron of the exchequer of Ireland on adoption of tory sympathies (1703/4). He died on 25 December 1705 and was buried in St Audoen's, Dublin, leaving a bequest to the poor of St Bride's parish, also in Dublin.
Donnellan married first Mary (d. 1684), daughter of Alderman John Preston of Dublin and widow of Rev. Edward Baynes of Dublin; they had three sons. He married secondly (March 1694), at St Audoen's, Martha Ussher, daughter of Christopher, the grandson of Sir William Ussher, clerk of the council in Queen Elizabeth's reign. They had two sons and four daughters, including Anne Donnellan (qv), the literary critic. After the deaths in 1751–2 of their mother and brother, the Rev. Christopher Donnellan, himself fellow of TCD, Anne's older brother Nehemiah challenged their bequests to Anne on legal technicalities and thwarted her inheritances. This Nehemiah Donnellan, of Nenagh, Co. Tipperary, and Artane Castle, Co. Dublin, was high sheriff (1733) and MP (1737–60) for Co. Tipperary. Another sister, Catherine, married the Rev. Robert Clayton (qv) of Dublin, later to be a bishop (Killala; Cork; Clogher). After the death of Baron Donnellan, his widow Martha married Sir Phillip Perceval (or Percival), an Irish civil lawyer. A portrait of Baron Donnellan, dressed in his judicial robe, was said to be in the possession of Dermot O'Conor Donelan of Sylane, near Tuam in 1885 (Anecdotes of the Connaught circuit, 55).