[1] William Sidney prominent politician and courtier during the reigns of Henry VIII and Edward VI, from both of whom he received extensive grants of land, including the manor of Penshurst in Kent, which became the principal residence of the family.

In 1556, Sidney served in Ireland with the Lord Deputy, Thomas Radclyffe, 3rd Earl of Sussex, who in the previous year had married his sister Frances.

Sidney played a large part in expanding the English administration in the country, which had shrunk over the centuries to the area around Dublin known as the Pale.

A second absence of the Lord Deputy from Ireland, on 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 temporising with the chieftain until Sussex reluctantly returned to his duties in August 1559.

About the same time, Sidney resigned his office of Vice-Treasurer of Ireland on his appointment as president of the council of the Marches in Wales, and for the next few years, he resided chiefly at Ludlow Castle, with frequent visits to the court in London.

[5] 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.

Sidney's time as Lord Deputy is controversial, due to the fact that the government extended its campaign against not only Gaelic military opponents in the field of battle, but also killings against the general population of the peasantry at large.

Sidney turned on the Hiberno-Norman Butlers in Ormond and Kilkenny, who had revolted against the opportunistic claims to their lands by Sir Peter Carew, an adventurer from Devon who pursued his entitlement with the blessing of the Dublin government.

In Antrim the MacQuillan of the Route and Sorley Boy MacDonnell were the chief fomenters of clan warfare, and after pacifying this northern territory Sidney repaired to the south, where he was equally successful in making his authority respected.

[8] Meantime Sidney's annual levy (the cess), which was designed to fund a central government militia, had caused discontent among the gentry of the Pale, who sent a deputation of eminent barristers to London to carry their grievances in person to Queen Elizabeth.

They were supported by several leading figures in the Irish Government, notably the Lord Chancellor of Ireland, Sir William Gerard.

[9] From his position on the Privy Council in London, Sidney used his influence in the bloody suppression of the Second Desmond Rebellion, which led to a great loss of life in Munster in the period 1579–83,[10] and ultimately to the plantation of the province with settler and planter families.

[2] His daughter, Mary Sidney, married Henry Herbert, 2nd Earl of Pembroke, and by reason of her literary achievements, was one of the most celebrated women of her time.