"These are two dogs."
Translation:Seo dà chù.
Two separate things are happening here. Firstly the dà causes lenition (i.e. the h). Secondly we do not pluralize after dà for historical reasons (millennia ago).
The n in coin has not been added in the plural - it was there in the first place (compare French chien, Latin canis, English canine). It has just fallen off in the singular.