William Downes, 1st Baron Downes PC (1751 – 3 March 1826) was one of the leading Irish judges of his time, and held office as Lord Chief Justice.
Downes was the second son of Robert Downes of Donnybrook Castle, MP for County Kildare, and his wife Elizabeth Twigge, daughter of William Twigge; he was a grandson of Dive Downes, Bishop of Cork and Ross and his wife Catherine Fitzgerald. He was related to the influential Burgh and Foster families and, through his FitzGerald grandmother, to the Earl of Kildare. He had an elder brother Dive, who took holy orders and died in 1798. Their father died when William was only three.
He graduated from the University of Dublin, was called to the Bar in 1776 and was elected a member of the Irish House of Commons for Donegal Borough in 1790. He was appointed a judge of the Court of King's Bench in 1792; on the murder of Lord Kilwarden in 1803, Downes succeeded him as Lord Chief Justice of the King's Bench in Ireland. He was Vice-Chancellor of the University of Dublin 1806-1816.
Downes was regarded as "the acknowledged father of the law". The low opinion held of him by his predecessor as Chief Justice, John Scott, 1st Earl of Clonmell, who called him "cunning and vain", can probably be disregarded, as Clonmell disliked and despised most of his judicial colleagues. In general Downes was respected for his integrity, although his manner was stern and intimidating, and it was said that he never laughed. According to Elrington Ball, after the death of Kilwarden it was generally agreed that only Downes was fit to succeed him. He was one of the few judges whom Daniel O'Connell could not intimidate. At the trial of John Magee for seditious libel in 1813, O'Connell's conduct of the defence was so intemperate that another barrister said that he should have been prevented from speaking; Downes said drily that he personally regretted not having prevented O'Connell from practicing law in the first place. On the other hand, Downes did let O'Connell speak in defence of his client at great length, and was severely criticised by the Chief Secretary for Ireland, Sir Robert Peel, for so doing.