O'Brien, Daniel (c.1630–1690), 3rd Viscount Clare , army officer, was the only son (there were also seven daughters) of Conor O'Brien (c.1605–70), 2nd Viscount Clare, and his wife, Honora O'Brien (d. in or after 1701), daughter of Daniel O'Brien of Dough, Co. Clare. On his father's side he was the grandson of Sir Daniel O'Brien (c.1580–1663), a brother of the 4th earl of Thomond (qv) and member of the confederate supreme council. Throughout the interregnum, he served in the Spanish army and at the royal court in exile, where he attracted the interest of Charles II. After the restoration of the monarchy, Donnough O'Brien (later Sir Donnough O'Brien of Dromoland), writing from London in December 1660, informed his mother: ‘here is our cousin Col. Daniel O'Brien, come out of Spain, and very much in favour’ (Inchiquin MSS, 6). As a reward for his services the family was included among the catholic nobility and gentry named in the king's declaration of 1660 and thereby restored to their former estates; further, his grandfather, Sir Daniel O'Brien, was raised to the peerage in 1662 as Baron Moyarta and 1st Viscount Clare. Under the Act of Settlement (1662) the O'Briens were restored to their principal seat at Carrigaholt in Co. Clare and to their estates, comprising 84,339 acres in the baronies of Moyarta, Inchiquin, and Bunratty. However, a proviso to restore quit rents to 1641 levels was omitted from the acts of settlement and explanation, and during the 1660s O'Brien spent a great deal of time petitioning to have the rents reset at 1641 levels, while trying to prevent revenue farmers from distraining his estate. As a high proportion of his property was unprofitable land, yielding a poor financial return, O'Brien became one of the most important horse breeders in the country, on occasion boasting that he had 16,000 acres set aside for his mares.
In 1667 the earl of Orrery (qv), fearing a French invasion of Munster, advised O'Brien to be watchful, particularly of the tories in Co. Clare. On the death of his father in 1670, O'Brien succeeded to the title as 3rd Viscount Clare. In 1674 he was commissioned as colonel of a new Irish regiment in the Anglo–Dutch brigade, possibly through the influence of his brother-in-law, Thomas Lennard, 15th Lord Dacre and future earl of Sussex, who was married to the illegitimate daughter of Charles II; he fought with the prince of Orange (qv) in Holland in 1675–6, though he was later deprived of his command when he was accused of spying for the French.
In 1681 Clare was drawn into the ‘popish plot’ paranoia in Ireland. Feigning conversion to protestantism, in January he informed the lord lieutenant, the duke of Ormond (qv), of a catholic plot to murder the English and subvert protestantism, promising to keep him informed of any further developments. Ormond did not trust Clare, however, believing him likely to have been involved in formulating such a plot, as he suspected the sincerity of his conversion. Clare overplayed his hand when in May 1681 he persuaded the grand jury of Co. Clare to send a petition to the king, advising him to hold a parliament in England to prosecute catholics. Ormond viewed this as an attempt to divide the protestant community and Charles was displeased at the grand jury's presumption in giving him advice. Upon examination the other members of the grand jury withdrew their support for the petition, leaving Clare exposed; he was removed from his role as justice of the peace and from his command of a troop of militia. In 1682 he apologised to Ormond and the king and sought a pardon for his actions.
When James II (qv) succeeded to the throne in 1685 Lord Clare was restored to favour. He was appointed lord lieutenant of Co. Clare, became a member of the Irish privy council, and in 1689 took his seat in the Irish house of lords. When Tyrconnell took control of the Irish army, commissions were issued to members of the catholic nobility to raise a force for the defence of the kingdom. At Carrigaholt, Clare raised the regiment that later became renowned as the Clare dragoons, noted for the yellow facings on their uniform. He also raised two regiments of infantry, which were commanded by his sons Daniel and Charles. The Clare regiments were initially stationed at Cork, where Lord Clare was appointed joint governor of the county and became notorious for his imprisonment of protestants. On 10 August 1689 he wrote from Cork to Donagh O'Brien of Dough, deputy lieutenant for Co. Clare, informing him that three French frigates, laden with arms and ammunition, were expected shortly at Kinsale. He instructed O'Brien to keep all protestants strictly guarded and to ‘leave not a young protestant in the county without straight confinement for which this will be your warrant’ (Inchiquin MSS, 16). Clare's regiment of dragoons fought at the Boyne in July 1690, where James's military abilities provoked his stern criticism.
Lord Clare married, in or before 1670, Philadelphia (b. 1644, d. in or after 1699), daughter of Francis Lennard, Lord Dacre; the couple had two sons – Daniel and Charles – and seven daughters – Margaret, Ellen, Honora, Catherine, Sarah, Mary, and Anne. The elder son, Daniel, left Kinsale for France with his regiment on 7 April 1690. When Clare died, in November of that year, probably at Carrigaholt, Daniel became 4th Viscount Clare. He fought at the battle of Marsaglia on 4 October 1693 but died shortly afterwards, at Pigernol, as a result of wounds received in action. He was succeeded by his brother, Charles (qv), 5th Viscount Clare. In May 1691 Clare and his sons were outlawed for high treason and William III granted the estate of 56,931 profitable acres to his favourite, Joost van Keppel, earl of Albemarle. In 1698 Albemarle disposed of his interest and, in March 1702, at the sale of forfeited estates, the lands of the O'Briens, viscounts Clare, were sold for the sum of £10,161.