One of the most important sources you can consult is the poem “Easter 1916” by William Butler Yeats. He is responding directly to the devastating violence that the British authorities inflicted against the Irish republicans during the six days of the 1916 Easter Rising or Rebellion. The eNotes Study Guide provides a thorough analysis of this seminal work (https://www.enotes.com/topics/easter-1916/). As it was published several years after the events Yeats addresses, it seems prophetic in his assessment of the “terrible beauty” that was born then, as the rebellion became a cornerstone of republicanism—something so dreadful that the Irish should never forget it.
There are numerous primary sources available that were produced at the time. One indispensable source is the 1916 Proclamation of the Irish Republic. A number of museums and archives in Ireland have originals of the proclamation, which were secretly printed and reproduced for distribution. The National Print Museum in Dublin, Ireland has a copy on permanent exhibit because it is considered the “most famous piece of printed ephemera” in Ireland. Patrick Pearse read it aloud on the steps of the General Post Office on Easter Monday, April 24, 1916. The British government later executed all seven signers of the proclamation.