Definition from Wiktionary, the free dictionary
- The original Irish Republican Army organisation, operating against British forces in the early 1920's, as distinct from the more modern group operating in the latter half of the 20th century.
- The term was not used contemporaneously, but is often appended to unit names when discussed in a historical context. Eg, the 4th Battalion, Dublin Brigade, would later be referred to as 4th Battalion, Dublin Brigade, Old IRA.
- The capital is sometimes not used for the letter O, without causing any difference in meaning.