Rebels occupied Clerys and the Imperial Hotel - located on the upper floors of the building on Sackville Street - at 9.30pm on Tuesday, April 25, the second day of the Rising.
A decision was made to set up outposts at the Imperial Hotel and the Metropole Hotel (located next to the GPO, at Prince's Street) in order to provide cover to the Volunteers' post office headquarters.
Heavy British shelling and the firing of incendiary bombs on Thursday and Friday led to a major blaze in the building.
The occupiers evacuated the building around midnight on Thursday.
Rebel Frank Thornton recalled: "We could have held these buildings for an indefinite period...but what beat us in the finish was the rain of incendiary bombs which kept falling all around...We remained until the ceiling of the first floor was falling."