I can't give you every fact about it, but I can tell you that Bloody Sunday was a day in the 70's on which Irish people protested against British oppression, and British soldiers killed fourteen of them and wounded many others.
The song is influenced by the events that happened in Dublin in 1920, where armed forces open fired into a crowd at a football match. 12 people were killed and hundreds were injured by the bullets and stampede.
A second incident in 1972, where British army troops killed 14 unarmed Irish citizens (7 of whom were only teenagers) during a civil rights demonstration in Derry (Northern Ireland). They called this day Bloody Sunday. And U2 did a protest song of it to spread awarness of the incident and convey peace to the audience. ;) Im currently doing a report and this is sum of the info i found :) X