The term Official IRA relates to one of the two elements of the Irish Republican Army - the other being the Provisional IRA - that emerged from the ideological split in the Irish Republican movement in 1969-70.
The reasons behind the split were many, but the main ones were the ending of violence for the IRA, and the ending of abstentionism for Sinn Féin. One reason which was never shown publicly was also that the IRA seemed to be drifting away from its Republican roots into Marxism. Many in the Official IRA later called the Provisional IRA the "rosary brigade" because of their Catholic and romantic nationalist ideology.
When the Provisionals (also called the "Provos") split from the Official IRA they took away a lot of experienced volunteers, which deprived the OIRA of some of the operational expertise they would later need during the months after Bloody Sunday which all but finished the OIRA as a paramilitary group. After the split and the failure of the post-Bloody Sunday campaign which lost them much support, the OIRA issued a ceasefire.
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