When Grandpa Abe Simpson smashes the hot tub, it not only breaks into pieces -- the pieces disappear entirely.
In the Republic of Ireland, the police are called the "Garda" and their vehicles are marked as such.
In th flashback the pub is shown to have corned beef and cabbage on tap. This is an american dish and not known in Ireland until recently and is made only in place catering to american tourists (in particularly American corned beef is not what Irish people would consider corned beef.)
In season seven's "Raging Abe Simpson and his grumbling grandson in The Curse of the Flying Hellfish", Abe relates how he and his platoon, who participated in the Battle of the Bulge, snuck into Germany to steal priceless art. But on this episode, it's stated that he'd spent his time in WWII in Ireland.
Marge and the kids seem to be all over the place while sightseeing, including Giant's Causeway which is way up north in Northern Ireland and Blarney Castles which is a 5 hour drive away, way down in the south of the Republic of Ireland.
This episode is predicated on Abe Simpson being based in Ireland during World War 2, but Ireland was a neutral country during the war and no US troops were stationed there.