Allusions are brief and often indirect references to historically, culturally, or mythologically significant characters, places, or occurrences. Allusions allow authors to set the stage upon which characters act and plots occur, but they require the reader to be informed about the relevant background in order to fully grasp their significance.
In Liam O'Flaherty's short story "The Sniper," the action occurs in Dublin during the Irish Civil War. The particular battle described in the story is set on the O'Connell Bridge and O'Connell Street.
O'Flaherty's use of this street is an allusion. O'Flaherty never elaborates on O'Connell Street's significance, but it is an important part of Dublin's geography and has a significant role in Ireland's bloody history. O'Connell Street was (and still remains) the main thoroughfare through Dublin, playing a large part in the city's economy. It was also the site of several major battles, both during the Irish Civil War and during the Easter Uprising six years earlier. Notably, in 1922, O'Connell Street was the site of a seven-day skirmish ended by point-blank artillery brought in by armored cars—a detail obliquely referenced by the armored car carrying the titular sniper's first victim.