Fleming, William (c.1611?–1642), 14th Baron Slane , politician, was second son of Christopher Fleming, 12th Baron Slane, a leading catholic lord of the Pale, and Elinor, daughter of Sir Patrick Barnewall of Turvey. The eldest son, Thomas, joined the Franciscan order, became an active member of the confederate association during the 1640s, and outlived his younger brother. William succeeded to the family estates in 1625 on the death of his father, and shortly afterwards married Anne, widow of Christopher Nugent (styled Lord Delvin), daughter of Randal MacDonnell (qv), 1st earl of Antrim, and Alice, daughter of Hugh O'Neill (qv), 2nd earl of Tyrone. The earl of Antrim accepted responsibility for the administration of the estates of his son-in-law, who was still a minor. On 30 October 1629 the crown granted a petition by Thomas that his younger brother William might enjoy the title, and the latter attended parliament in 1634–5 and again in 1640–41.
Slane only rose to prominence after the outbreak of the Ulster rising in October 1641, although during the preceding months he had sat on a number of important parliamentary committees. When news of the rising reached Dublin, Slane and the other Pale lords professed their loyalty to the administration, but the lords justices proved reluctant to arm any catholics. On 16 November the parliament appointed Slane as a member of a delegation to initiate contact with the Ulster rebels. The proroguing of the parliamentary session by the lords justices the following day thwarted any attempt at a compromise settlement. Two weeks later, at a meeting on the Hill of Crofty in Meath, Slane, Nicholas Preston, 6th Viscount Gormanston (qv), and the rest of the Pale leadership formed an alliance with the Ulster Irish, led by Sir Phelim O'Neill (qv). Slane was given the responsibility for raising troops in the barony of Slane, and his name appears among the signatories of a petition sent to the king, explaining the actions of Irish catholics. By March 1642, after a series of military reversals, Slane approached James Tuchet (qv), 3rd earl of Castlehaven, in an attempt to arrange a truce with the king. The lords justices, however, arrested Castlehaven, and in April the advance of Sir Charles Coote (qv) (d. 1642) the elder through Meath forced the Pale lords to abandon their headquarters in Trim. In September 1642 an expeditionary force from Dublin captured Slane castle, the family home.
On a political front, Slane had been prominent in moves to establish confederate governmental institutions during the summer of 1642, and he attended the first general assembly in Kilkenny from 24 October. He died the following month, shortly after being declared an outlaw on 17 November. His great friend and ally Viscount Gormanston died eight months later, and according to one contemporary source, both were ‘mighty grieved in general of all men’ (Gilbert, Contemporary history, i, 52). The two deaths deprived the Pale lords and gentry of effective leadership, a major factor in their decline as a political force within the confederate association. Slane was succeeded by his eldest son Charles, who became an active confederate military commander, fled abroad following the Cromwellian invasion, and died in foreign service. The second son, Randal, recovered the family estates in the 1660s.