>But we have been discussing how the Christian celebrations of halloween are a lot younger than Christianity itself,

Halloween wasn't a foundational observance of Christianity, so in that sense it's "younger." But we do have strong records of its origins, where it fell on the calendar, what basic liturgical observances there were, and how these customs evolved over time, step by step. The Celts really didn't have a say in it.

>and we don’t really know how old Samhain is, right? We only have a record of it from the 9th century but that’s because we don’t have really have a written record of Gaelic Ireland before then.

It's the perfect thing to project upon – which is what happened in the 19th century: The era of romanticized history that gave us the tall tales about pagans and figures like Christopher Columbus. And we can see how accurate those were (i.e. not).

And we know through a large number of anthropological studies the idea that "it was always this way" is always flawed without an historical record to compare it to. Customs change over time, and many of the customs that we associate with Samhain today were not even mentioned until well after the genuine pagan Celts had all died out. There is only so far one can honestly extrapolate back.