Thousands of homes in County Donegal, Ireland, built from concrete blocks, are damaged by extensive cracks and crumbling that occurred a few years after construction. Recently, research has shown that pyrrhotite oxidation triggering internal sulfate attack (ISA) is the cause. In this study, samples from the strip foundations, the rising blocks, the outer and inner leaf of one undamaged reference home and three affected homes are investigated. As these four structural components differ by concrete quality, exposure condition or both, their effect on ISA is investigated. All three damaged homes contained pyrrhotite in the aggregates, while it was absent in the reference home. ISA in the foundations is in an initial state. It has progressed further in the rising blocks and is most advanced in the outer leaf. Whilst carbonation limits the effects of ISA in the inner leaf, further pyrrhotite oxidation will create expansion leading to ongoing deterioration.