To learn more about The Troubles, the violent thirty-year conflict that pitted Catholic nationalists and republicans against Protestant unionists and loyalists in Northern Ireland, you will find short, useful summaries at the following links.
Wikipedia
https://en.wikipedia.org/wiki/The_Troubles
BBC
http://www.bbc.co.uk/history/troubles
The Irish Story
http://www.theirishstory.com/2015/02/09/the-northern-ireland-conflict-1968-1998-an-overview/#.WfALhRNSzeQ
Alpha History
http://alphahistory.com/northernireland/
