In the 12th century, Ireland was conquered by the English, brutally enslaving the country and the Irish for the next 800 years. In the 1500s, the Church of England changed from Catholic to Protestant, (after King Henry VIII wanted a divorce to have a younger wife). The Irish people's refusal to convert caused discrimination against them by the English to double ten-fold.
In the late 1840s, Ireland was plagued with a potato famine that lead to a million Irish people starving to death due to the anti-Catholic British regime refusing to help. 3,000,000 Irish fled abroad to escape starvation and tyranny, most of them coming to the United States, such as the Stabler family.
Ireland has been beset by violence known as "The Troubles", a conflict between the Provisional Irish Republic Army against the British army and loyalist paramilitary groups, most notable the Ulster Volunteer Force.
Peace was eventually declared in 1998 with the Belfast Agreement. The ceasefire left many IRA insurgents out of work, so they became mercenaries. One example of a former IRA man becoming a hit man was Liam Connors.
The history of organized crime in the United States started with potato famine-refugee Irish immigrants in the early 1800s before the Italian Mafia or Russian Mafiya came into existence.