As this is originally an Irish place name you would need to consult a book or expert on the origin of such names. There is a good chance that, like many place names, the original meaning has been lost. The meaning "sweet valley" or "beautiful valley" seems to have originated in a song called Sweet Vale of Avoca written by the Irish poet Thomas Moore:
It is possible that Avoca would mean "sweet vale" in Gaelic and Moore knew this, but it is just as possible without evidence that the people who interpret Avoca as meaning "sweet valley" took this idea from the song, and it is not the original etymology of the word. We need to find an expert on Irish place names to find out.
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