Sorry for lapsing into English, and I know this isn’t the question, but people might be interested to know the origin of this difference. Originally, lenition after a preposition + an indicated the dative case, while eclipsis after a preposition + an indicated the accusative case, and there was a difference in meaning. The dative indicated a stationary location, while the accusative indicated motion toward a destination. (This difference is also found in German, and in Latin except with the ablative in place of the dative.) Thus in Early Modern Irish you could find “ins an chathraigh” for “in the city (in der Stadt, in urbe)” vs. “ins an gcathraigh” for “into the city (in die Stadt, in urbem)”. But even as early as Keating the distinction is beginning to fall apart, and today it’s purely dialectal and has no semantic difference at all.
