The InterRail website
https://www.interrail.eu/en/interrail-passes/one-country-pass/ireland is confusing in relation to Ireland passes being valind in Northern Ireland as it states 'The Emerald Isle is a small island of dazzling contrasts with its own soundtrack, from lively Dublin bars to rugged rocks, impressive mountain ranges, and kissing stones. Visit Belfast, Blarney, Limerick, and Waterford.'
The Belfast reference clearly implies cross-border validity. However, firther down it states '
Can I cross the border?
Not with this Pass, as it’s all about exploring one country from top to bottom. Want to cross the border? Go for a Global Pass.'
It is only when you click 'Learn More' that it states 'The Pass is valid for travel only in Spain and cannot be used to travel from the Netherlands to Spain'.
It therefore appears that the 'one country' reference is mainland Europe.
Perhaps someone needs to tell InterRail that the island of Ireland includes more than one country.