William Tisdall was Jonathan Swift's friend.
Their friendship however suffered a long rupture, due to their rivalry for the affections of "Stella", whom Swift had known since she was a child in the household of his employer Sir William Temple, and who moved to Ireland in 1702 to be closer to Swift. Tisdall wrote to Swift in 1704 announcing that he intended to propose to Stella, and Swift wrote in reply to dissuade him. Swift's letter was courteous enough in tone- he stressed the practical difficulties of such a marriage, since Tisdall was not a rich man, and the Church had not yet found him a salaried position - but he privately expressed his disgust at Tisdall as an "interloper", and their friendship cooled off. It is not entirely clear whether or not Tisdall proposed to Stella, but if he did she refused.
After Stella's death in 1728, the two men were reconciled, and Tisdall witnessed Swift's will.