Fire appliances are at the scene and the road has been closed between Agnes Street and Carlisle Circus.
The Courthouse, linked to the Crumlin Road Gaol by an underground passage, was designed by Sir Charles Lanyon and completed in 1850.
It was closed in 1998 and later sold to local investor Barry Gilligan for £1.
Mr Gilligan plans to redevelop it as a tourist attraction and hotel.
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