From Proto-Indo-European*melh₂-(“to grind”). Various explanations exist for how Brythonic ended up with an a root vowel and Goidelic with an e.
Watkins claimed that the Brythonic was from an athematic present (where a zero grade could be found), and the Goidelic would be from a thematic present. However, Schumacher points out that a laryngeal-final athematic present would be expected to produce a weak verb, for which no trace in Celtic exists.[1]
Schumacher thinks that the Brythonic root variant was derived from the zero grade and was the sole present allomorph in Proto-Celtic. He further explains the Irish stem mel- as generated by some secondary analogical means, via some expectation that the present should be in the e-grade. Schumacher cites two possible sources of analogy. One of them was *gʷeleti(“to graze”), which had similar phonological shape and shared a semantic field. The other influence would be the subjunctive, which in strong verbs is expected to have the e-grade which also often occurs in the present stem as well.[1]
Matasović is agnostic on which root vowel came first, merely remarking that the Brythonic reflects the zero grade and Goidelic the e-grade.[2]
Darling integrates both the *mal- and *mel- stems in a single present paradigm, starting with a simple e-grade thematic present *melh₂-e-ti. Whenever the thematic vowel was *e (in the 2nd person, and in the 3rd-person singular), it would be coloured by the laryngeal into *a, which would then feed Joseph's Law (*eRa > *aRa, where *R is a resonant), and turn the root vowel into *a; the Brythonic present stem can be derived by leveling the a across the present stem. Meanwhile, the forms with thematic vowel *-o- would not be a-coloured and thus the root vowel remains as *e; the Goidelic forms would be derived from leveling the e across the present stem.[3]
↑ 1.01.1Schumacher, Stefan, Schulze-Thulin, Britta (2004) Die keltischen Primärverben: ein vergleichendes, etymologisches und morphologisches Lexikon [The Celtic Primary Verbs: A comparative, etymological and morphological lexicon] (Innsbrucker Beiträge zur Sprachwissenschaft; 110) (in German), Innsbruck: Institut für Sprachen und Literaturen der Universität Innsbruck, →ISBN, pages 470-472
