Here are the EN TTS(1) voices: [Only registered and activated users can see links. ]. They'll work in all US/EU nuvi 2x5 devices. It they don't show up in Tools>Settings>Language>Voice of your 255 it's for some other reason, perhaps a modified region 16.

No Irish sorry (not sorry). Garmin has never made one for any flavour of TTS, surely because nobody outside of Ireland would understand it. So as not to discriminate, there's no Scottish voice either likely for the same reason (D'ye know where the hospital is Jimmy?"). There's just 6 in English - 2 Pommies, 2 Yanks and 2 Aussies - that's it mate.

The Aussie female voice, Karen, (unfortunate name these days, eh?) is the most popular English GPS voice in the world. Karen Jacobsen, in life a pretty blonde with a friendly cheeky smile, also has an angelic singing voice and was paid a pittance to sit in a sound booth for endless hours to record thousands of words and phrases early in her career as a singer and voice-over artist. She's known world-wide as "the GPS Girl" and downunder as the Australian Siri on iPhones. She's also a native of my home State, Queensland, and if you don't choose her as your new favourite GPS voice, you'll be finding dead pigeons in your cistern for the next few years Boyo.
No pressure though ....
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[Only registered and activated users can see links. ]