GPO also stands for General Post Office. Perhaps the most famous GPO is that on O'Connell Street in Dublin, which was occupied by the rebels of the 1916 Rising, and used as their headquarters. The choice of the GPO as the rebel's main target perhaps shows their naivete: it was certainly a prominent public building, but of no real strategic value. Significantly, they failed to make any attempt to capture Dublin Castle, which was the real centre of communication for the British troops.
The GPO was rebuilt after the rising, and still serves as the capital's main post office. It also houses a famous bronze statue of The Death of Cuchulain.