Equinox is the date (or moment) some astronomical alignments in Ireland mark as being auspicious. Not many, mind you, but some, like the cairn on Loughcrew or the two passages of Knowth, a sort of super-alignment with quadruple significance. Though the actual alignment of Knowth is disputed, it might be a lunar alignment or not an alignment at all. Let us find out a bit more about the equinox.
The Irish Calendar
The equinox is far less obvious an astronomical event than the two solstices, celebrated in Ireland and also the subject of astronomical alignments. It is like the equinox, which occurs in-between the winter solstice and the summer solstice, and vice versa, twice a year. However, it is just one event, as the spring and autumn equinox happens at different dates, but are for all intents and purposes identical events.
Taking place around March 20th and September 22nd, the equinox is the moment when the plane of Earth's equator passes the center of the Sun. That is, it's when the Earth's axis is in an exact ninety-degree angle to an imaginary line between the centers of Sun and Earth. This happens twice a year when the vacillation of Earth's axis reaches a neutral middle point. Doctor Who might say it all is a wibbly-wobbly timey-whimey thing...
This is the moment when the tilt of the Earth's axis is neither inclining away from the Sun nor inclining towards the Sun. A moment only, but generally speaking, we call the whole day this happens "equinox." "Equinox" is a word from the Latin, literally translating as "equal night". It indicates that day and night have roughly the same length on this day.
Of course, we are talking cosmic scale here, so "roughly" is as good as it gets and an exact description of the phenomenon would fill whole books with loads of scientific material and mathematical additions. While the cosmic workings are incredibly precise, human ability to grasp them lacks a bit.
So, for the layman: the equinox is the day when the Earth sort of "stands upright", everybody gets the same amount of sunshine and that period is as long as the night. It happens twice a year.
In agricultural societies, the equinox would mark the seasons and also frame certain activities and expectations.
Take hens, for instance. They (very broadly speaking) need twelve hours of daylight to lay eggs in earnest. The main time to get eggs is, you guessed it, between the spring and autumn equinox. And this might also explain why we have an Easter egg, as some people think that the Christian feast of Easter is, after all, a version of the feast of the equinox. The same claims have, however, been made for Saint Patrick's Day... though that is far less likely (after all, Easter was already well established in Saint Patrick's days).
Though the equinox is regarded as the astronomical start of spring and autumn, the celebrations of same are not occurring on the equinox in Ireland. Spring, for instance, starts on Saint Brigid's Day, February 1st, or Imbolc.
So was the equinox a big thing? Not really, it seems. It is more of a marker one notices in passing, though there are a number of old festivals associated with both the spring equinox and autumn equinox. The timing may, after all, just be a coincidence (as in Patrick's case).