The IRA which fought for Irish independence 100 years ago was not quite the same organisation that exchanged equipment and training with the Palestinians.

The old IRA won independence for most of Ireland, specifically 26 of 32 counties - 6 other counties, 4 of them majority Protestant, decided to stay part of the UK as Northern Ireland. At the time, the richest and most industrialized area of Ireland was in the northeast, such as the Belfast dockyards which had built the Titanic in the previous decade. Although Britain certainly did not want Irish independence (and had brutally repressed the Easter Rising in Dublin in 1916, only a few years before), it perhaps didn't fight to keep it with the same fanatical zeal as the French in Algeria decades later, for example. Overall casualties for both sides were low, but marked with atrocities and guerilla-style warfare. Several factors contributed to Britain's willingness to withdraw, such as the negative impact it was having on Britain's international image, the king's displeasure with the conduct of British forces, the escalating expense of the war, and a sense that it would be extremely difficult to pin down the IRA. In any case, Ireland had always been a problem province, and the south was viewed as being backwards and impoverished anyway, while the loyal Protestant north was economically prosperous and of considerable military importance, both as a shipyard and for covering Britain's Atlantic flank. So, there were many factors as to why the War of Independence succeeded, but the IRA's competence was certainly a key factor. They and their hit-and-run tactics succeeded where many other rebellions had failed.

The IRA were not completely dormant in the following decades, but their activity levels certainly reduced. The last major IRA operation was the border campaign in the 1950s and 1960s, which ended in abject failure. By the 1960s, the IRA were increasingly involved with Marxist philosophy, and decreasingly involved with blowing things up.

Now, during this period, Northern Ireland's Catholic / Nationalist population lived in a borderline apartheid state, facing harsh discrimination in politics, education, jobs, policing, justice, housing, healthcare, transport and just about any other aspect of life you care to imagine. This has been at least partly attributed to a so-called "siege mentality" amongst the Protestant / Unionist population, which felt under threat both from the south of Ireland and Nationalists within the north. During the late 1960s, efforts were made in Northern Ireland to coordinate peaceful protest with the goal of ending discrimination against the Nationalist community; the single largest group was called NICRA, the Northern Ireland Civil Rights Association. They had several demands, such as a policy of "1 man, 1 vote" and an end to gerrymandering that artificially amplified Protestant political influence; an end to discrimination in the allocation of government jobs and housing; and the abolition of the Special Powers Act, which gave the government extreme leeway to bypass civil rights in the name of maintaining public order, frequently abused to target Catholics.

The marches organized by NICRA were not well received. They were attacked by Protestant civilians and even police (who were overwhelmingly suspicious of Catholics, being largely Protestant). Several contentious marches led to days-long rioting, and in the worst days of the violence thousands of Catholics were driven out of their homes, often by the burning of entire neighbourhoods. During this time, the IRA was hesitant to get involved. Their new Marxist ideology held that the sectarian divide was merely a tool of the upper classes to divide and subjugate the working classes, and so they broadly wanted to avoid fighting against working class Protestants, who they viewed as potential allies. To Catholics being burned out of their homes, this was of little consolation, and there was bitterness towards the IRA and their perceived cowardice, with a common joke being that IRA stood for "I Ran Away". Given this situation, several senior IRA figures who were more inclined to militantism decided to break away, forming a new group: the Provisional Irish Republican Army. Initially, it was a pretty much 50/50 split. However, the Provisionals grew rapidly, both from recruitment and defection from members of the original group, now called the "Official IRA". The Provisionals seized de facto control of large areas of Belfast and Derry (officially called Londonderry, but against the preference of its inhabitants). When the British Army was deployed to Northern Ireland to restore order and reign in the undisciplined police, their bumbling incompetence and heavy-handed tactics alienated the Catholic community and led to a surge in Provisional recruitment. By 1972, the Provisionals were firmly established as the dominant faction of the IRA, and have remained so ever since. In modern Britain and Northern Ireland, "the IRA" pretty much always refers to the Provisionals unless specified otherwise. The Officials continued to exist, but with relatively little influence.

The 1970s was the bloodiest period of the Troubles, particularly the early 1970s. The (Provisional) IRA's goal was to swiftly drive out British forces from Northern Ireland, and in this they failed. However, they forced the removal of many discriminatory laws by making clear to Britain the cost of oppression, and so the legal situation for Catholics did improve.

By the late 1970s it was clear that conventional war was not going to win a United Ireland. Instead, the IRA decided on a long war of attrition, and by the early 1980s also decided to try their hand at electoral politics, under the slogan "a ballot box in one hand and an Armalite rifle in the other". This was most vividly displayed in the IRA hunger strikes, one of the most emotive and memorable moments of the whole conflict, when imprisoned IRA member Bobby Sands (along with several others) went on hunger strike to protest prison conditions, while also successfully running for a seat as a UK Member of Parliament. He would die shortly after, and the Catholic community was thoroughly swept up in the drama of the occasion. Sands was viewed as a martyr and over 100,000 people attended his funeral.

The IRA (and its political wing, Sinn Fein) had now proven that they were here to stay, and although they would never command the majority support of the Catholic community during the conflict, the IRA were respected and admired by many; even people who did not actively support them were hesitant to turn them in to the government, and tacit support (like car rides or a place to stay when needed) was commonly offered to "connected men". In the 1980s, the IRA became better equipped, receiving shipments of weapons from Libya as well as funding from the Irish diaspora in the US and elsewhere. They even came within minutes of assassinating British Prime Minister Margaret Thatcher in the mid 80s.

However, the high profile "supergrass" investigations of the 1980s seriously weakened the IRA's internal cohesion, ultimately leading to a less centralized cell structure. As the Troubles went on, British security forces became more familiar with the tactics used by the IRA and the success rate of their operations decreased, with one key exception being the IRA's South Armagh Brigade.

By the 90s, the situation was a stalemate and war-weariness was settling in. Backroom negotiations intensified between the IRA and the British and Irish governments, but ceasefires merely punctuated continued violence. By this point, the IRA was concentrating on targets of economic importance, with the highest profile incidents being the destruction of Manchester city centre, and the bombing of Canary Wharf, London's main financial district. The eventual outcome was the Good Friday Agreement, which promised that Northern Ireland would only join the Republic of Ireland if a majority of the population supported it; so far, there has not yet been a vote.

The consensus on their efficacy depends on what you mean by efficacy. The Provisional IRA was undoubtedly one of the most capable and dangerous insurgencies in modern history. They wielded great political influence, and were well-armed, funded and organized; the British government dealt with them as equals, and the British military was thoroughly impressed with their capabilities, noting that they surpassed any Unionist paramilitary in their level of success and organisation, and that some IRA units were even comparable to British special forces in their capabilities. Toby Harnden's "Bandit Country" describes the relationship between IRA and British military in very readable terms, if it is this aspect in which you are most interested.

And yet, the IRA did not achieve their goal of a United Ireland. Whether they might in the future lies outside the scope of AskHistorians, but by the formal end of the conflict in 1998, Northern Ireland remained a part of the United Kingdom.

The IRA also fought to protect Catholics from Unionist violence. The argument has been made that IRA escalation ultimately made Catholics less safe by leading to a chain of reprisal killings between Nationalist and Unionist paramilitaries, although several academics (Rona Fields, Jonathon Tonge and Timothy Shanahan to name a few) view the mass burning of Catholic homes at the start of the Troubles as an attempted pogrom.

Interestingly, one could reasonably claim that the IRA's most enduring legacy is a non-violent one: though initially enjoying only limited success in Northern Ireland and the Republic of Ireland, Sinn Fein (the IRA's electoral offshoot), is now by far the biggest political party on the island.