Swiss meringue buttercream tastes too buttery

Aside from what the other commenters have said, did you add the butter too quickly? I know when you make regular buttercream you have to beat the butter for a long time, as this will reduce the 'buttery' flavour. Just a thought for next time?

Although some European butters can contain more butterfat than American styles, and some of it is cultured- the big difference between an Irish butter and an American product has to do with what the cows ate. Irish cows are grass fed, almost exclusively- because grain is simply too expensive to feed. They even eat a fermented grass during winter, which is best known as silage.
American dairy cows eat a lot of feed, and not nearly as much grass. That is what makes the big difference in how the butter tastes. I'm not sure how you would change the flavor profile, other than to add in perhaps an additional flavor element, perhaps a good quality vanilla, or even a citrus based flavoring. I'm thinking a Fiore di Sicilia would be a good compliment, if you could find it. In the future, since you find the European, or at least Irish butter a bit too buttery tasting in this application, you would want to use a more neutral American butter.

Butter, though a packaged seemingly standard product, does vary by season, farm, country. Could be the Irish butter has higher butterfat and/or flavors reflecting where the cows grazed.
If the buttercream reminds you of buttered popcorn, maybe there's also a higher amount of salt from that butter.
Use the buttercream where the taste won't be unpleasant, with strong other flavors to balance it, and a low amount or no added salt.