In 1556 he went to Ireland with the lord deputy, the 3rd Earl of Sussex, who in the previous year had married his sister Frances Sidney; and from the first he had a large share in the administration of the country, especially in the military measures taken by his brother-in-law for bringing the native Irish chieftains into submission to the English Crown. In the course of the lord deputy's Ulster expedition in 1557 Sidney devastated the island of Rathlin; and during the absence of Sussex in England in the following year Sidney was charged with the sole responsibility for the government of Ireland, which he conducted with marked ability and success. A second absence of the lord deputy from Ireland, occasioned by the accession of Queen Elizabeth, threw the chief control into Sidney's hands at the outbreak of trouble with Shane O'Neill, and he displayed great skill in temporizing with that redoubtable chieftain till Sussex reluctantly returned to his duties in August 1559. About the same time Sidney resigned his office of vice-treasurer of Ireland on. being appointed president of the Welsh Marches, and for the next few years he resided chiefly at Ludlow Castle, with frequent visits to the court in London.
In 1565 Sidney was appointed lord deputy of Ireland in place of Sir Nicholas Arnold, who had succeeded the earl of Sussex in the previous year. He found the country in a more impoverished and more turbulent condition than when. he left it, the chief disturbing factor being Shane O'Neill in Ulster. With difficulty he persuaded Elizabeth to sanction vigorous measures against O'Neill; and although the latter successfully avoided a decisive encounter, Sidney restored O'Neill's rival Calvagh O'Donnell to his rights, and established an English garrison at Derry which did something to maintain order. In 1567 Shane was murdered by the MacDonnells of Antrim, and Sidney was then free to turn his attention to the south, where with vigour and determination he arranged the quarrel between the earls of Desmond and Ormonde, and laid his hand heavily on other disturbers of the peace; then, returning to Ulster, he compelled Turlough Luineach O'Neill, Shane's successor in the clan chieftainship, to make submission, and placed garrisons at Belfast and Carrickfergus to overawe Tyrone and the Glynns.
In the autumn of 1567 Sidney went to England, and was absent from Ireland for the next ten months. On his return he urged upon Cecil the necessity for measures to improve the economic condition of Ireland, to open up the country by the construction of roads and bridges, to replace the Ulster tribal institutions by a system of freehold land tenure, and to repress the ceaseless disorder prevalent in every part of the island. In pursuance of this policy Sidney dealt severely with the unruly Butlers in Munster. At Kilkenny large numbers of Sir Edmund Butler's followers were hanged, and three of Ormonde's brothers were attainted by an act of the Irish parliament in 1570. Enlightened steps were taken for the education of the people, and encouragement was given to Protestant refugees from the Netherlands to settle in Ireland.
Sidney left Ireland in 1571, aggrieved by the slight appreciation of his statesmanship shown by the queen; but he returned thither in. September 1575 with increased powers and renewed tokens of royal approval, to find matters in. a worse state than before, especially in Antrim, where the MacQuillins of the Route and Sorley Boy MacDonnell were the chief fomenters of disorder. Having to some extent pacified this northern territory, Sidney repaired to the south, where he was equally successful in making his authority respected. He left his mark on the administrative areas of the island by making shire divisions on the English model. At an earlier period he had already in the north combined the districts of the Ardes and Clandeboye to form the county of Carrickfergus, and had converted the country of the O'Farrells into the county of Longford; he now carried out a similar policy in Connaught, where the ancient Irish district of Thomoxid became the county Clare, and the counties of Galway, Mayo, Sligo and Roscommon were also delimited. He suppressed a rebellion headed by the earl of Clanricarde and his sons in 1576, and hunted Rory O'More to his death two years later. Meantime Sidney's methods of taxation had caused discontent among the gentry of the Pale, who carried their grievances to Queen Elizabeth. Greatly to Sidney's chagrin the queen censured his extravagance, and notwithstanding his distinguished services to the crown he was recalled in September 1578, and was coldly received by Elizabeth. He lived chiefly at Ludlow Castle for the remainder of his life, performing his duties as president of the Welsh Marches.
Sir Henry Sidney was the ablest statesman charged with the government of Ireland in the 16th century; and the meagre recognition which his unrewarded services received was a conspicuous example of the ingratitude of Elizabeth. Sidney married in 1551 Mary, eldest daughter of John Dudley, duke of Northumberland, by whom he had three sons and four daughters. His eldest son was Sir Philip Sidney, and his second was Robert Sidney, 1st Earl of Leicester; his daughter Mary married Henry Herbert, 2nd Earl of Pembroke, and by reason of her association with her brother Philip was one of the most celebrated women of her time.
See Calendar of State Papers relating to Ireland, Henry VIII - Elizabeth; Calendar of the Carew Manuscripts; J O'Donovan's edition of The Annals of Ireland by the Four Masters (7 vols., Dublin, 1851). Holinshed's Chronicles, vol. iii. (6 vols., London, 1807); Richard Bagwell, Ireland under the Tudors (3 vols., London, 1885); Calendar of Ancient Records of Dublin, edited by Sir JT Gilbert, vols. i. and ii. (Dublin, 1889); Sir JT Gilbert, History of the Viceroys of Ireland (Dublin, 1865); JA Froude, History of England (12 vols., London, 1856-1870).