It's not Gaelic at all, that much is clear from the formation. The closest in Gaelic is anthracs which is simply anthrax using Gaelic alphabet but more on that later...
There's Andreu, Catalan (Spain) form of Andrew but that's as close as it gets. Other than the Greek root meaning 'carbuncle' that developed into the word 'anthrax'. Perhaps your parents saw or heard Andreu somewhere, thought it was pretty and either mispelt it or deliberately changed the d to th? That would be my best guess. Far preferable to the 'carbuncle' option...;o)
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