Ash, Thomas (1660?–p. 1735), diarist and defender of Derry, was born in Killylane, near Muff, Co. Donegal, before his family moved to Corrinerin, later known as Ashbrook, Co. Londonderry. One of twenty-four children of John Ash, he was the third son of John's second wife, Sarah (maiden name unknown), who died in 1668 in childbirth, having had five sons and five daughters. The family was connected to many other planter families, including that of David Cairnes (qv); Jane Browning, wife of Michael Browning (qv), was probably his sister, as was (possibly) an ancestor of John Brown (qv) (d. 1873). Thomas was sent to school in Derry in 1671, and looked after his stepmother's property in Co. Antrim from 1674 to 1684, when his father died. He married (13 July 1686) Elizabeth, only daughter of Thomas Becke of Magilligan; she died in 1688 and their two daughters died young.
Ash was one of the protestant gentlemen attainted by the parliament of James II (qv) in 1689; he became a captain in a regiment that garrisoned Derry city during its siege. He served with distinction, but is remembered chiefly for his diary of the siege, which has become one of the prime sources for historians, providing much detail about events and conditions within the city. When the siege ended, he found his farm burned out. He was transferred to Col. Lance's regiment and took part in further action. He was sent on a flanking mission, so witnessed, rather than took part in, the battle of the Boyne. He almost died of a fever contracted while campaigning, and on his recovery took part in skirmishes round Sligo, notably against Sir Teig O'Regan. On 6 April 1692 he married Elizabeth, only daughter of Hugh Rainey, a wealthy merchant and ironfounder, and lived for a time with his father-in-law in Magherafelt, Co. Londonderry. Ash was high sheriff of Co. Londonderry in 1694, was elected an alderman of Derry in 1704, and was a lieutenant-colonel in the county militia from 1725. On the death of Hugh Rainey (1707), Ash was executor of his will, by which Rainey, who believed that his survival in a shipwreck had been due to divine providence, left half his estate, about £4,650, to charitable purposes. To carry out Rainey's provisions, Ash borrowed money on his own securities, and in 1710 spent over £7,000 on an estate near Downpatrick, Co. Down, which was to provide an income to support a charity school in Magherafelt, still in existence in 2002 as the Rainey Endowed School. Ash also established a number of almshouses in Downpatrick, intended for widows of clergymen of the diocese of Down and Connor. His charities, and possibly also providing for his thirteen sons and four daughters, greatly reduced Ash's wealth, and his heir is said to have been in great poverty. His wife died 8 November 1728. In his seventy-fifth year the old man wrote an account of his many relatives and of his own life; though a number of pages were lost before it was transcribed and published, it is of considerable interest, especially to genealogists. The date of his death is unknown.
Ash was a member of the Church of Ireland; his second wife was a presbyterian, and one of their sons became a presbyterian minister. According to Thomas Witherow (qv), Hugh Rainey had intended the charity school to be for presbyterians. but an eighteenth-century statute put it under the control of the archbishop of Armagh.