How much of this is down to "snowbirds"?
I've seen reports that Canadians are turning their backs on Florida - and selling their properties there - and heading elsewhere, and that Lisbon (along with the rest of that country) was actively trying to attract them, but their primary reasons (ICE, and a state legislature going out of their way to drive them elsewhere) don't really apply to US snowbirds. Bali, Colombia and Thailand would also fit. If this effect is down to that then 2025 is probably a one-off - they will have spent winter 2024/25 in Florida, Arizona or whatever, winter 2025/26 outside the US but only their departures will have been registered because they have not returned yet.
Of course Ireland is less of a destination for that kind of tourism.