Shanice could be an elaboration of
Shan, an Anglicised form of
Sian, which is indeed the Welsh form of
Jeanne. Welsh and Gaelic aren't the same language--though they could be related; however I'm blanking on whether they definitely are or not, sorry--but many people lump Irish, Scottish, and Welsh together out of ignorance/confusion.
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As for your name's pronunciation, as it's a made up elaboration, I would say it could be pronounced in any way your parents preferred. SHAN-is (like
Janice) is as perfectly valid as, say, shan-EES or something like that imo.
iranda