I don’t know that it absolutely eliminates it. A lot can happen in 200+ years, and people can go back to Ireland as readily as they can try to emigrate to the new world. Plus, I don’t believe it was ever established that he was a direct lineal ancestor, merely an ancestor—so he could have been some family member’s brother who went to America leaving the rest of the family in Ireland. But good memory!
As I recall, that Revolutionary War Cap was established as part of continuity _after_ it had already been established that young Steve Rogers grew up on the Lower East Side in an immigrant community (Jack Kirby had tossed out the visual in a pin-up, but the character wasn’t put into a story until decades later). So it was always an iffy connection, but was too much fun not to use.
I don’t think we’re talking about the Revolutionary War Cap (which was a mistake that became canonical, but that’s a whole other story.) but rather Steve’s Revolutionary War era namesake, who was a part of Kirby’s “Madbomb” storyline. The main villain’s ancestor had suffered a slight at the hands of this previous Steve Rogers, one he was sword to revenge.