An in-depth examination of the acquisition of grammatical gender has not previously been conducted for Irish, an endangered indigenous language now typically acquired simultaneously with English, or as L2. Children acquiring Irish must contend with the opacity of the Irish gender system and the plurifunctionality of the inflections used to mark it, while also experiencing early exposure to the majority language and variability in amount and consistency of adult input in Irish. Data were collected from 306 participants aged 6-13 years, including information on home language background which allowed children to be categorised as being from homes which were Irish-dominant, bilingual, or English-dominant. Novel measures of receptive and productive use of grammatical gender were developed to test children's understanding and production of gender marking. A standard multiple regression conducted which accounted for 39.5% of the variance showed that language background was the strongest predictor of accuracy in marking grammatical gender assignment and agreement. The later stages of acquisition of semantic and grammatical gender have not previously been investigated in Irish, and the implications for researchers, policy makers, educators and parents are discussed.