"...fairly predicts the demise of Irish Celtic"

Has there ever been a language known as "Irish Celtic"?
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keep and kept etc are particular examples of a vowel change from long e to short as an indication of tense.

Also shows up in, for example bleed/ bled, or dream/ dreamt. And there are other analogous vowel changes for the same purpose eg run/ran.

But the alternative, and more "regular" way of doing it coexists and overlaps, with the vowel staying long and "ed" being added at the end. And children, being pretty sharp, notice that regular form and apply it, which is why they say things like "I runned" or "I keeped". And that's how those forms creep into the language, and often take over from the older form.

It's a very curious language. But then they all are.