* Scottish Gaelic: The most common word is goid (pronounced roughly like "goyd"), but this is actually the verb "to steal." To make it refer to a person, you'd need to use a descriptive phrase, such as duine a ghoid ("person who stole"). A more direct translation might be gadach (pronounced roughly like "gah-ach"), though this often implies a dishonest or deceitful person more generally than just a thief.
* Irish Gaelic: The word for "to steal" is goid. Similar to Scottish Gaelic, you'd need a descriptive phrase like duine a ghoid ("person who stole") to refer to a thief. Fear goid ("man of stealing") would also work. There isn't a single perfect equivalent to "thief."
Therefore, there's no single word equivalent. You need to use a phrase describing someone who steals.