The idea of postboxes, according to Stephen Ferguson’s wonderful The Irish Postbox, may have come from the seventeenth century denunciation boxes common in Italian city states such as Florence and Venice. Citizens were encouraged to post anonymous reports of criminal activity. The situation of this postbox, outside Dublin Castle may carry some resonances then, through misty-eyed glasses. This one bears the relatively unusual insignia of Edward VII, with the gorgeous large swirling ‘E’, and has thankfully been spared of an appendage, so far.
Dame St itself was the scene of a postal dispute in the 1850s. The Commercial Buildings on College Green were threatened with the replacement of their postal collection room (and man to bring the post up to the GPO) with a mere post-box on the street, collection time: 5 pm. Members wrote to The Freeman’s Journal:
It is preposterous to expect that a pillar letter-box in the street, to be emptied in all sorts of weather, will be entrusted with the valuable correspondence of mercantile firms.
The dispute lasted three years and the postal room was eventually reinstated. 
Reference:
Stephen Ferguson (2009) The Irish Postbox: Silent servant and symbol of the state, Dublin.
Wide and Convenient Streets 