Hibernia as a national personification representing Ireland appeared in numerous cartoon and drawings, especially in the nineteenth century...
as depicted in frequent cartoons in Punch, a magazine outspokenly hostile to Irish nationalism, Hibernia was shown as "Britannia's younger sister"...She is an attractive, vulnerable girl.[4] She is threatened by manifestations of Irish nationalism such as the Fenians or the Irish National Land League, often depicted as brutish, ape-like monsters. Unable to defend herself, Hibernia is depicted turning to the strong, armoured Britannia for defense. Wikepedia

So it is somewhat ironic to now have the Irish flag flying behind her.

The GPO was built from 1818 to 1821.