Fall Equinox is the time when the day and night are of equal length opposite of Spring Equinox. There is not much known about how the Irish Celtic/Gaelic peoples celebrated this time of year but it is very likely similar to other Indo-European practices that center around agriculture and harvest activities. Many pagans today celebrate the neo-pagan holiday of Mabon that focuses on Autumn earth elements and the crops, (apples and corn in particular), that are ripe for the picking. Following the summer celebration of Lughnassadh this is the time to start preparing for the dark half of the year and the coming cold of winter, by harvesting the last of the year's crops. Grain, wheat, blackberries (and other fruits), making corn dolls and the colors of autumn (Orange, Red, Brown and Green) are very popular symbols of the harvest and many events take place outside around the fire with offerings.
Despite the fact that there is little known about the celebration practices in Irish Celtic times I can imagine this was a time to pause to celebrate and honor the abundance of the growing season and the last of the harvests. This would be an understandable time to thank and honor the Gods of agriculture, weather (favorable weather especially), and abundance, as well as the Earth for providing through the hard work of people who's lives centered around the agricultural cycles of the year.