Why do my grandparents call potatoes irish potatoes ?

Potatoes were once a popular crop that were grown in Ireland on many farms.

Perhaps they call them that to distinguish them from sweet potatoes. And plain potatoes are closely associated with Ireland.

What people call things sometimes changes over time. It can also vary depending on where you lived while you were young and learning what to call things, because things can have different names in different places.

When shopping malls first arrived in my city back in the 1960s, they were called shopping plazas. My mother still calls them that. We didn't have a sofa when I was a kid. We had a chesterfield. The shoes you wear to the beach, the ones called flip-flops? When I was a teenager, they were also called thongs. If you grew up in the US south or in the UK, maybe you call it a filling station. I've never heard anyone here in Canada call it that. It's a gas station.

Look up the Irish potato famine.