How do you write Katrina in Gaelic?

There isn't a single, universally accepted way to write "Katrina" in Gaelic. Gaelic refers to both Scottish Gaelic (Gàidhlig) and Irish Gaelic (Gaeilge). The transliteration would depend on which language you're aiming for and the specific pronunciation you wish to emphasize. There are several possibilities, and none are definitively "correct" without more context.