Existing coupled fire–structure analyses for ships generally rely on idealised models with regular breaches and uniform material properties, neglecting heat-affected zone (HAZ) degradation at the breach edges (a term used here in a generalised sense), while the coupled stress mechanism remains poorly understood. In this study, large-eddy simulation with the Fire Dynamics Simulator (FDS) is combined with a sequentially coupled thermal–structural analysis in Abaqus. Equal-area breach models with a degraded HAZ band quantify the individual and coupled effects of HAZ degradation and breach-shape irregularity. The HAZ raises the stress concentration factor (K_t) by approximately 16% and shifts the peak stress location from the geometric corners to the weakened annular band. Under equal-area conditions, the combined HAZ and irregular polygon effects elevate K_t by about 71% relative to a rectangular breach without an HAZ, about 8% higher than the multiplicative superposition estimate. This synergy is significant only for highly irregular breaches with sharp re-entrant corners, whereas weakly irregular shapes conform to the independent-product assumption. An approximately linear relation between the shape irregularity index and K_t over nine equal-area shapes supports first-order screening estimates. The findings provide a screening framework for prioritising repair of fire-damaged ship cabins that remain within the elastic regime.