This eclipsis after "ar an" is a Connaught and Munster usage
On the other hand, lenition-aspiration occurs "In Ulster Irish after ar an (on the), ag an (at the), as an (from the), faoin (under the), tríd an (through the), ón (from the), leis an (with the)"
Collins Dictionaries (2011-07-28). Easy Learning Irish Grammar (Collins Easy Learning Irish) (Irish Edition) (Kindle Locations 320-322). HarperCollins Publishers. Kindle Edition.