Irish Scribes - Why Didn't They Need to Do It?

Christianity arrived in ireland round the sixth century. And they started to write in the latin alphabet instead of em in ogam, which is a bit like runes, but different shape. Taves a bit of bother. Yes, it saves on ink. Smart move, irish scribes. But why did they need to do it? They didn't understand the words as immediately as a latin native speaker would. If you listen to a foreign language being spoken, you've got no understanding of at all. It's just a string of sounds a and if you see an unfamiliar alphabet, you wouldn't be able to kind of discern what the words were in it without the brakes