As the etymology of the name shows, the name literally means "great tide," and in theory could be applied to boys or girls. The ocean has both "masculine" and "feminine" qualities after all!
― Anonymous User 7/9/2014
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In regards to the male/female debate, Dylan (pronounced DUL-an) is attested in Welsh mythology as a male name. Dulan (pronounced DIL-on) however appears in the Old Welsh charters in the Book of Llan Dav (c.1150). So with Anglisised pronunciation of Dylan - the spelling is male but the pronunciation was once female.
My grandfather's name was Daniel and my dad's name is Donal. My name is Dylan. My grandmother who was Scottish told me Donal is Scottish for Daniel and Dylan is Welsh for Daniel. I don't have evidence for it but I've always believed it and spread the info to those who ask.