"Republic of Ireland and Northern Ireland."
Translation:Poblacht na hÉireann agus Tuaisceart Éireann.
As ever, context is king. Per §7.7 of Graiméar Gaeilge na mBráithre Críostaí,
Fágtar an t-alt ar lár roimh Éireann i dteidil reachtúla áirithe; Dáil (Seanad, Raidió Teilifís, Iarnród, Tuaisceart ) Éireann, agus i seanleaganacha: oileán iathghlas Éireann ; … feara Éireann ná a mná. Ní hionann an bhrí a bhaintear as an teideal polaitiúil Tuaisceart Éireann agus as an ngnáthabairt tuaisceart na hÉireann.
As descriptions of political jurisdictions, Tuaisceart Éireann is the proper choice for contrasting with Poblacht na hÉireann.
I came here wondering the same. It appears that Tuaisceart is not an adjective as in the English ("Northern") but rather it's a noun ("North"): https://www.teanglann.ie/en/gram/Tuaisceart
So "Tuaisceart Eireann" is NN, Noun-Noun, with the second noun in the genitive form. There's discussion above about how "Northern Ireland" is usually written "Tuaisceart na hÉireann" but when referring to the title-of-the-state-of-Northern-Ireland for some reason the "na" is elided.