Not an exception, because there is no such rule.
The singular definite article an causes eclipsis after certain prepositions. (most commonly ag, ar, as, chuigh, faoi, le, ó, roimh, trí, thar and um). Other prepositions, like de, do and i cause lenition after an. But there are prepositions that don't cause any change, and prepositions that require a genitive noun.
(and then there are different rules for different dialects!).