It'd seem to me the overhand way would also make better use of the arm as a lever - think of an ice-pick downward stab with a dagger for comparision - plus it sort of adds the benefit of gravity to the equation, for whatever that now is worth.
The Irish of them olden days were a bit sui generis mind you. Early Modern English soldiery were still a bit puzzled as to why the Irish cavalry insisted on fighting overarm with light spears without stirrups, and the generally rather antediluvian character of the native warfare...
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