Viscount FitzWilliam, of Merrion in the County of Dublin, was a title in the Peerage of Ireland.

The earldom became extinct on his death in 1667, but he was succeeded in the barony and viscountcy by his younger brother William FitzWilliam, the third Viscount.

His son Richard, the sixth Viscount, was a member of both the Irish and English Privy Councils.

They are recorded in Ireland from the beginning of the thirteenth century, and through acquisition of large estates in Dublin, became within a few generations one of its dominant families.

Their Dublin estates passed by inheritance to the Earl of Pembroke, whose descendants are still substantial landowners in the area.