She was born in Belfast in 1840 and immigrated to America in 1856. From my research it looks like the Irish language was falling out of official use in Northern Ireland in the latter half of the 19th century. It's clear that she knew English well enough, but how commonly was Irish used while she was living in Ireland and would a young person have learned it/spoken it at home? Any insight would be appreciated.
Someone from Belfast could be either Protestant of Catholic. If the former, Irish Gaelic our have been nearly out of the question (there is a slim by unlikely chance that she might have known at least some Scots Gaelic). If she were Catholic, if is also unlikely that she would have known Gaelic as the language was ceasing to be spoken in that area and an urban setting would have made it even more improbable that she would have known it. She would have been familiar with some words, and her syntax and pronunciation would have been affected by Gaelic - as it is in the Hiberno-English dialect to this day, but it is unlikely that she would have been fluent in the language.
edit thanks to /u/myothercarisawhale