There are people that will tell you there is a difference between Sinn Fein, the IRA, and the "Real IRA". They are all different manistifications of the same multi-headed monster, basically anarchist and nihilist terrorists, with even less ideology than the Muslim terrorists. Their original purpose was to rag on the British Crown, and English Parliament, then later on the US, whom they thought to be far too chummy with the British. All anti-Americanism is at its base, nothing more than an extension of anti-British sentiment.
But at heart, these Irish terrorists are only an extension of the basic Irish character, a people who would go out of their way to invent adversity if there was not enough of it already in their lives.