The Agia Irini Gorge runs all the way from the village of Agia Irini to the coastal resort of Sougia in the south. The best thing you can do if you want to walk this gorge is to take the bus from Sougia to Chania, or take a taxi, and get out at Agia Irini. It is less spectacular than the Samaria Gorge, but also much less busy :). The walk takes about 3 to 4 hours, so make sure you bring plenty of water.
At the start of the walk there are enough places where you can take a rest in the shade, but the last one and a half hour as you get closer to Sougia the shade is more difficult to find. In theory there are water taps on the route, but these don't always work.
You start your walk in Agia Irini. Take the second of the two tracks (to the right of the green and white sign with the map of Agia Irini Gorge), go down and keep the river to the right. You will walk past a taverna and over a bridge. Just after the bridge you will see a sign that leads to the Sotiros Hristou chapel which has frescoes dating from 1357 showing sinners being punished in hell.
The walk through the Agia Irini Gorge is free (since 2016 there is a 2 euro entrance fee and this might change in the future). Make sure you have good walking shoes on, because the terrain get rough sometimes. Most of the walk is alongside the river and some of it through the (dry) riverbed. The nature in the Agia Irini Gorge is lush, with very tall plane trees, cypress trees and many flowers and herbs. You can sometimes see the rare kri kri goat here. The cliffs are 500 meters high at some places and 10 meter wide at its narrowest point. On the route are benches under trees and places with toilets.