Astronomer of Dublin
(The). The head of the chief rebel of Dublin, set on a tall white-painted stake on the highest point of Dublin Castle, where it remains till it falls to decay or is replaced by the head of a greater rebel. The Irish say: “God send to Dublin many more astronomers.”
His head is poled high Upon the castle here, Beholding stars as though he were A great astronomer.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894