He was born in Aberdeen, Scotland. He appeared in 1676 in Dublin as "physician to the state and my lord-lieutenant" (according to Sir John Hill, quoted in Culloden Papers, London, 1865), and was elected one of the fourteen fellows of the Dublin College of Physicians in 1677. From 1681 to 1687 he was president of the college, and again in 1690–3, in 1696, 1698, and 1706. He was elected in 1692 to the Irish House of Commons for Killyleagh and sat there until 1695. He was then elected for Mullingar, and held that seat until 1713. Dun accompanied King William III to the Battle of the Boyne.
Dun was evidently a physician in Dublin, and had great social influence. He was the friend and medical adviser of Archbishop King (1650–1729), and of many other influential people.
On his death in 1713, Dun left his personal library to the college.
In 1815 Sir Patrick Dun's Hospital was founded and named in his honour, by the College of Physicians, noted physicians William Stokes and of Robert James Graves served in the hospital. The hospital was closed in 1986, when it became the venue for civil marriage ceremonies.
- Leigh Rayment's historical List of Members of the Irish House of Commonscites: Johnston-Liik, Edith Mary (2002). The History of the Irish Parliament 1692-1800 (6 volumes). Ulster Historical Foundation.
- Dictionary of National Biography