collins dictionary: The eve of a particular event or occasion is the day before it, or the period of time just before it. ...if someone was asked to work xmas eve, they wouldnt automatically assume it's that night or that evening, 99% will take eve as the day before ..and trathnona is evening, oiche is night
It is a Pagan holiday, it is shortened for "All Hallow's Evening". The way to shorten "Evening" is to drop the "v" and the "ing" and it becomes "E'en". The following day, All Saints Day, or All Hallows Day, is the end of Harvest. The traditions on that day are even deeper than those of our modern-day Halloween. On Halloween, the idea behind disguising yourself, was to hide from Death. I'm not positive on the exact details, but I know that there is a tradition with rock carving, where an individual would carve their name into a rock, toss it into the harvest fire on All Hallows Evening, and the next day (All Hallows Day), if you found the rock with your name on it among the embers, you would live through the next harvest season. If not, you could expect death. Morbid, but extremely fascinating.
Actually, I've not come across hiding from death. The opposite is the case, at least in the very old ways I know of in Connacht.
The disguise is worn if you're out and about to blend in with the Sí (the 'good neighbours' or Tuath Dé who normally live beneath the sí), and encourage the dead to follow you as they do the Sí.
The Sí are leading your dead ancestors to your home so they can check you out, see what you've done with what they left you, pass on omens and messages to you and such. The Sí lead the dead with lights, people dressed as the Sí carrying lights (candles inside carved out turnips to stop the wind blowing them out carried strung up on poles) can help lead your dead ancestors too. The dead ancestors, the helpful Sí (and helpful people dressed as Sí) would be rewarded with something nice, usually something sweet with honey and milk. Failure to reward could bring very bad luck on the house and people in it because you'd have insulted not only the Sí but your own ancestors too (and people trying to be 'helpful'). You'd leave a light in your window to help the Sí but mostly your dead ancestors find you. You might have moved a few times since great, great, great biddy passed on, so the old girl needs a bit of help tracking you down.
The Sí and the dead can leave the under-world at this point in the year because time is suspended temporarily (so the Druí back then could adjust their lunar calendar) and the rules changed, not even the days were named. This festival lasted more than one night back then.