The Irish Republican Army was a militant wing in Ulster that declaired war on England because it considered itself Irish and sought union with the Republic of Eire who had rejected British Rule successfully. Ulster remained divided between those who considered themselves British, and those who considered themselves Irish occupied by British Forces.
For a long, long time the IRA basically used Terrorist acts against the UK Mainland to try and force Westminster to let it have independance. Mostly these were limited to shooting sprees and car bombs which went right through the Cold War era and into the late 1990s before a peace deal saw a provincial Government for self rule under Westminster formed, with members of both the Loyalists and the Nationals take positions of power.
Her Britannic Majesty made history earlier this year by visiting the Republic of Eire, the first time since Irish Independance, which was not really gained on ammicable terms from Westminster took place during the Great Depression era.
Now the Queen has gone to Ulster to meet someone probably responsible for the death of Lord Mountbatton....Her Husband...I believe a closer relative still, went with her