Well, that’s not actually true. Brythonic and Gaeilge languages were using a Latin based scripted before the English language existed. In Scotland, the original Celtic language was Pictish which was Brythonic. They spoke Gaelic in many parts of Scotland due to an Invasion by the Irish Scotti tribe. Gaelic has never been phonetic, Welsh (I am Welsh) and Cornish, however, are. English royal and government oppression over the Celtic nations existed of course and still does to some extent. The Welsh language was brutalised by English policy. However the phonetics and script the languages use have little to do with English oppression. They Anglicised many words in the languages, especially names of people and places. Cardiff, for example, is just a bad English way of saying the Welsh ‘Caerdydd’, but the Irish names in question here haven’t been Anglicised. As I said I do not know enough about the USA to comment further on why these names can be pronounced compared to others, I just know there is a largely white community that regard themselves as Irish so the names may be popular.