The place names are all proper names, thus they are definite by virtue of being proper names. However, there are some place names which take the definite article in genitive, but not in other forms.
ÉIRE, i.e. Ireland. It is Éire in the nominative, but after simple prepositions, it still takes the dative form, Éirinn. (This is a literary usage. In dialects, the dative form has ousted the historically correct nominative form, thus Éirinn even in nominative.) But the genitive takes the article: na hÉireann.
ALBA, i.e., Scotland. It is Alba in the nominative, Albain in the dative. But na hAlban in the genitive.
GAILLIMH, i.e., Galway. It is Gaillimh in the nominative, na Gaillimhe in the genitive. Note that “in Galway” is i nGaillimh. But in Munster Irish, you do see sa Ghaillimh (sa = ins an = in the). I don’t know why – Galway is not even in Munster.
EAMHAIN MHACHA, i.e. the legendary Emain Macha. It is na hEamhna Macha in the genitive, at least if we are to believe Father Peadar Ua Laoghaire.