I assume you're talking about the Provisional IRA so I'll answer regarding them.

Simple answer, they didn't wear uniforms however there was a 'standard' for various operations depending on their nature.

Raids on army and police bases: Typically the IRA would wear boiler suits, boots, 'work' gloves and a balaclava during such operations, this was both to prevent the dropping of evidence but also tp allow for quick changing when escaping the conflict area.

Sniping attacks(rural): Rural IRA snipers most often wore camo jackets or green coats to blend into their surroundings, balaclavas typically werent worn as it was unlikely the army or police would have a chance to see them.

Urban: In urban combat the IRA would wear civilian clothes, transitioning from long coats and jeans in the seventies to track suits in the nineties although the use of boiler suits was common for assassinations.

Ceremonial: IRA Ceremonial dress comes in two forms, the black jumper, trousers and boots with black leather gloves and a black beret (the gloves and beret of a killed volunteer would often be given to the family) or a more military suit with the gloves and beret, balaclavas wear often worn to hide their identities.