There's no single "Gaelic accent". Gaelic refers to a family of Celtic languages, including Irish, Scottish Gaelic, and Manx. The spelling of a name like "Murtha" would vary depending on which Gaelic language you're referring to and even then, the pronunciation and thus potential spelling, could vary regionally within that language.
To get a potential spelling, you'd need to specify which Gaelic language you mean and even provide more context (e.g., is this a surname with a known origin?). Then, a Gaelic speaker familiar with that region's dialect could offer a more accurate and informed transliteration.