The Irish Civil War and subsequent conflicts were so divisive of Irish society that it is now necessary to refer to the original IRA as the "Old IRA" in order to distinguish it from other groups which have since claimed to be the Irish Republican Army. While most Irish people would accept that we owe our independence to the original IRA, none of the organisations which have styled themselves "the IRA" or "Óglaigh na hÉireann" since 1926 have been able to claim the support of more than a tiny minority of the Irish people for their campaigns.
By "the original IRA" I mean the group which was first formed under that name, to fight the British in the Irish War of Independence. This group was an amalgamation of the Irish Republican Brotherhood, the Irish Volunteers and other groups, and its military commander was Michael Collins. Its mandate to prosecute its guerrilla war derived from Dáil Éireann, the independent Dublin parliament formed by elected Sinn Féin deputies who seceded from the Westminster parliament following the 1918 general election.
This incarnation of the IRA effectively came to an end with the signing of the treaty in 1922. The organisation was split more or less down the middle by this agreement, although it was ratified, albeit by a small majority, by the people's elected representatives in Dáil Éireann. Most IRA fighters followed their commander Collins and formed the regular army of the new political entity, the Irish Free State. A significant minority, however, led by Eamon de Valera, refused to accept any deal short of a full republic. These were known as the irregulars, or die-hards, and their occupation of Dublin's Four Courts heralded the start of the Civil War.
The irregulars, who continued to call themselves the IRA, came off second best in this divisive conflict, and eventually became reconciled to the idea of gaining independence incrementally. The bulk of the movement became de Valera's Fianna Fáil, which is today Ireland's largest political party, and engaged with the apparatus of the new state, in order to transform it, a process culminating in the 1937 Constitution of Ireland, which defined Ireland as an independent political entity. A rump of die-hard die-hards did not follow de Valera, and remained comitted to establishment of a 32 county republic by any means necessary, claiming their mandate from the 1918 general election, even though this political dispensation was fading into history.
All subsequent incarnations of the IRA have derived from this root, although they have been through numerous splits, changes in leadership, and most of the personnel over the years have drifted back into the democratic fold. The organisation most people recognise as the IRA today, the Provisional IRA, continues to identify itself with the IRA who fought the War of Independence, but only tenuous links can be drawn between the two. The Provos' first chief of staff, Séan Mac Stíofán, was an Englishman who became converted to the Republican cause while in prison during the 1950s. Its current leadership became politically active during the civil rights movement of the late 1960s.
Supporters of today's IRAs (e.g. Provisional IRA, "Real" IRA, Continuity IRA), of course, will attempt to show that they are the direct descendents of the original IRA. The fact that most of the original members of the Old IRA became members of political parties such as Fianna Fáil and Fine Gael, or entered Ireland's regular army and police force, is discounted on the basis of ideology. To today's IRAs, the members of the Old IRA, the people who actually fought for our independence, are in fact traitors to the republic, except for the tiny minority who did not become reconciled to the democratic process.
Very few of these people are still alive and soon there may be no survivors of the original conflict. Invariably, however, these people refer to their activity with the "Old IRA". They find it necessary to use this term, as the name "IRA" has been thoroughly sullied by groups which have appropriated it.