Merrion Castle was a castle situated about 300m south of the present-day Merrion Gates, to the south of Dublin city centre. Built in the early fourteenth century, it was from the sixteenth century to the early eighteenth century the principal seat of the Fitzwilliam family, who acquired the title Viscount Fitzwilliam. After the Fitzwiliams moved to Mount Merrion House in about 1710 the castle fell into ruin, and it was demolished in 1780, though there were remains visible as late as 1837. No trace of Merrion Castle survives today. It was located opposite Merrion Gates, on the site of St. Mary's Home and School for the Blind. Its location, and the modern site of St.Mary's, can be seen on historical maps, including the six-inch (1829-1841) Ordnance Survey of Ireland maps.[1]