OK, my two cents worth, which ain't much.
Some Irwin's sailed well, not the CC tubs though, they were built with interior
volume in mind, they weren't bad sailing, they just didn't sail as well as a comparable Pearson, of course the similar sized Pearson didn't have quite as much interior
volume. Either would handle what was thrown at them, they just did it in a different way, depends on what your looking for.
As for build quality there is no comparison, Pearson was always known for the quality of their boats, compared to the boats in their segment of the market they always had a good reputation. Irwin's not quite as good in build quality and that varied from year to year and model to model, depends on which one and what period it was built in, Pearson's were more consistent.
Depends on what you need and how much work it needs.