NavList:

Bob:

"Midpoint" is interesting to me because it raises the question of "middle of what?"  It could be the middle of the number of days between "solstice" and "equinox" (however someone might define those terms), or it could be the middle in terms of ecliptic longitude (keeping in mind that the sun's speed along the ecliptic isn't constant), or it could be something else entirely.  Sometimes the word "astronomical" is used as an "appeal to authority" without mentioning what kind of star-measuring is being used.

I don't know much about ancient astronomy, but I'm left with the impression (supported by your link) that the Iron Age(?) Europeans who observed cross-quarter days tended to mark progress through a year by noting the sun's azimuth at rising on a horizontal circle.  If I were doing that, I'd be sorely tempted to bisect the angle between the equinox and solstice azimuths and use that as my definition of a "cross-quarter day."  In CelNav terms, I'd look for days where the sun's amplitude is half of what it is at a solstice.    The catch with this idea is that you'd end up with different dates for different latitudes, especially for latitudes greater than 45º (which describes all of Ireland).