Statuto paid more than €200m ((£139m) for the 118-bedroom hotel, which has been described as a world record in terms of value per bedroom.
It's the biggest deal in Italy since Statuo bought the Hotel Danieli in Venice from Starwood last year for $177m (£123m).
The transaction leaves Quinlan, which is headed up by former Dublin tax inspector Derek Quinlan, with Four Seasons properties in Dublin, Budapest and Prague, along with London's Maybourne Group.
The group acquired Maybourne in 2004 when it bought the former Savoy Group for £750m. Its hotel include the Savoy, Claridge's and the Berkeley.
By Angela Frewin