How are the Ogham stones inscribed?

Question

I earlier asked How to convert Old Irish Latin script to Ogham? and am not quite complete with the answer. I have a similar question still trying to dig into how to build an Ogham generator. But instead of asking directly how to build one, I am going to ask about how the original Irish people (or Welsh or whatever it may be) converted their thoughts into Ogham.

Not to be such a complicated ask, I am mainly trying to get at how they transcribed their words into Ogham. Say they were speaking Early or Old Irish, or Middle Irish even, or some Welsh, or Latin even. How do you then take your desired speech/words and convert them into Ogham? Are they doing it by the sounds they are making or hearing from their voice? Sounding it out? Then mapping the sounds to Ogham? Or do they have an intermediate spelling system where they first write it in that, then convert to Ogham (like speech -> latin -> ogham)? Probably they sound it out I would guess. Is there any research on this?

Or are there examples of sort of "rosetta stones" in Ogham that show Ogham next to Latin script, showing that they (at least sometimes) took Latin letters and mapped them directly to Ogham?

The goal of this question is to help paint the picture of how a transliterator could work. Should it be based on the sound of the words, or the latin letters, or a mixture? That's what I'm thinking deep down. But the question can stay focused on the research related to Ogham and how it was actively written (the techniques). Not techniques like how they held their utensil, but how they mapped their "words in their heads" to Ogham. If nothing exists, then what is a good hypothetical?

Answer 1

Since you talk about the "original" Irish people, I'm going to assume you mean the original inventors of the script, who spoke an early Q-Celtic language in the fourth century CE. (Also note that Celtic is not my area of expertise, so others should comment to correct me if I say something wrong.)

At this point, the twenty "classical" Ogham letters mapped quite directly to the phonology of the language. We can't be entirely sure, but linguists seem to agree that there was pretty much a one-to-one mapping between phonemes and letters—there are no phonemes reconstructed for Primitive Irish that aren't represented in Ogham, and while there's one letter whose original pronunciation isn't certain, there don't seem to have been any excess or unnecessary letters either.

Over time, Primitive Irish evolved into Old Irish, and the phonology of the language changed; some new letters were invented, others were re-assigned to new values, and the use of Ogham steadily declined until it died out completely and was replaced by the Roman alphabet. But if your goal is to inscribe a stone marker in the fourth-century style, you can just straightforwardly map each Primitive Irish phoneme to its appropriate character.