Although the Old Irish article in is standardly described as a marker of definiteness, it also co-occurs with indefinite nouns. This phenomenon has long been known in the literature, but thus far even an adequate descriptive account of it has proven elusive. This article advances two claims about the distribution of in. First, indefinite referents introduced by in become the focal center of the discourse. Second, in co-occurs with both definite and indefinite noun phrases because it is a signal to the addressee to retrieve or establish a mental representation of the referent. Although the distribution of in is unusual within Indo-European, it is actually predicted by the reference hierarchy of Dryer (2014). The Old Irish article is thus of particular importance for our understanding of the typology of article systems and referential marking.