What was the significance of bloody sunday in ireland

Well, there have been several, all of them about Catholic Ireland vying for freedom from the laws and rule imposed on them by Protestant England, many of which restricted rights and treated the Irish like second-class citizens in their own country. Ireland, remember, is not part of the United Kingdom. On each occasion, Irish were attacked and killed during demonstrations by British military forces.

Catholics (Roman, Greek, Coptic) have endured a great deal of persecution in Ireland, England, the United States, Nazi Germany, the Holy Land (yes) and many other parts of the world across the centuries. Catholics are also a strongly united group all across the world (which can be very surprising, given how wildly varying theological beliefs between Catholics can be, but that's a different topic). These two things create a shared history across geopolitical culture amongst Catholics. When you add the fact that their religion is built with a deep reverence for tradition and its preservation, it's little surprise that they carry so much history with them.