"They don't use the euro in England, they use the pound."
Translation:Ní úsáideann siad an euro i Sasana, úsáideann siad an punt.
The simplest form of "the rule" is that you don't use an article for the "big 5" - Éire, Albain, Sasana, Meiriceá and Ceanada.
The more detailed version of the rule is that once you get to countries that you have to look up in the dictionary anyway, because you don't know what the Irish for them is, you should check whether or not they take an article.
(And things get a bit more complicated in the genitive too, but that won't come as a surprise!).
In Irish, many countries require the use of a definite article - An Fhrainc, An Ghearmáin, an Iodáil. As i+an is sa, you use sa with these countries, and sa becomes san before a vowel sound - sa Fhrainc, sa Ghearmáin, san Iodáil.
Countries that don't take a definite article, don't use sa - i gCeanada, in Albain, i Meicsiceo.