Since it’s given as An tAontas rather than An t-aontas, its translation should be “The Union” rather than “The union”. I’d think that something like an tAE would be the Irish analogue for “the EU” (but I’m not sure about how articled initialisms are treated in Irish). As a US citizen, I’d tend to think of “The Union” as one of the belligerents in the 1861–1865 war.
EDIT: The translation above has been updated to read “The Union”.
Northern Ireland unionists often refer to the Union, i.e. the United Kingdom of Great Britain and Northern Ireland.
Why do we use 'aontas" rather than 'cumann'? Aontas seems to be specific to political unions.
We use 'ceardchumann' for trade union. I see the 'cumann' in there for 'group of people with shared interests' or even 'marriage', but don't the countries in a union have shared interests too? (in principle, I'm not being politico)
Although the Students Union is 'Aontas na Mac Léinn in Éirinn'.
So is it just a convention then?
A masculine noun in the nominative/accusative case (singular) that starts with a vowel is prefixed by "t" when it follows the definite article "an" and the "t" is not capitalised.
e.g. an t-asal, an t-aontas
an aisling, an iris as they are feminine nouns.