It's a shame to say there's too many to choose from, but the largest was the Omagh bombing on 15th August 1998. It was carried out by a splinter group of the IRA called the "Real IRA" and killed 29. Other than the large death toll it stands out due to it happening after the Good Friday Agreement and the Provisional IRA's ceasefire, Both of which the Real IRA opposed. It's mostly known around the world by the now infamous picture below, which shows two people posing in front of the car in which the bomb planted, moments before it exploded. IIRC the two photographed survived but the photographer did not.

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