An investigation has been launched at Leinster House after flooding in various parts of the building following Wednesday's torrential downpour.
The floods brought Government Buildings to a communications standstill, with both telephone lines and computer systems affected.
Frantic efforts are now underway to repair damage to 1,500 lines in the Leinster House exchange.
The roof of the new £25m extension saw water leak into the lift shaft and flood into the building.