What the game manual tells us:

The Green Mire Warrior was a mysterious Sho spearman who was known for his bravery. He would fight powerful foes with reckless abandon, and would often laugh while doing so, even when his wounds were grievous and death seemed certain. He is a central figure in many sho stories and songs, but even so, he remains an enigmatic figure, clad in armor the color of polished jade, wielding his yari with a master's skill.

The rumor for the quest can be purchased from Brother Gonun in Shoushi for 14p. The directions lead you to the Green Mire Grave, north of Shoushi at 27.8S, 71.6E. The rumor also mentions a nearby Mosswart Camp where you may find an Ice Tachi.

The Grave is full of Mosswarts, including, Mudlurks, Barkers, Shamans and Chiefs. In addition, you'll encounter some Drudge Slaves, a Dark Wisp and a Ghost Wisp.

Map: Green Mire Grave

Once inside, you'll need to find (surprise, surprise) ...a few keys! Each of the four wings of the dungeon must be explored before you can retrieve your prize.

The first key, the Worn Key, is found in the South Wing. Follow the passage around to the right until you come to a large room with Mudlurk Mosswarts and a Drudge Slave. The key is on the Slave. You'll also find a note lying on the floor next to a corpse.

Rumor: Personal Note

The second key, the Short Key, is found in the North wing. Head down a corridor until you come to a room with a few Mosswarts. The key is on the Barker Mosswart. You'll also find a note lying on the ground with a few hints about exploring the dungeon.

Rumor: Piece of Parchment

You'll next need the Short Key to open a door in the upper level of the East wing, where the Yari can be round. The East wing can be confusing because you begin with a maze-like area. From the entrance area, keep heading straight until you find yourself in a small room with a door to your east, south, north and west. Take the door to the south, then keep east, then north. You're looking for a passage down a long hall. You'll know you're heading in the right direction when you spot another note lying in the ground.

Rumor: Piece of Parchment

At the end of the long hall, you'll find a ramp going up with a locked door at top (Resistance 154). Use your Short Key (north wing) to open it! Go up another ramp and make a left at the T. Continue heading up until you reach the large open room. The Large Key is found on a Mudlurk Mosswart, guarded by a Dark Wisp! You'll also be fighting Mosswart Shamans.

While you're there, explore the room a bit. You'll find two chests in an alcove. The right one can be opened by the Worn Key from the south wing and contains the Green Mire Yari, a Tachi and a Yoroi Breastplate! In the middle of the room, you'll also find a coffin where the Green Mire Warrior was laid to rest.

Next, head back down to the main room (portal recall if you like) and head towards the West wing.You'll need both the Large Key and the Worn Key to complete this section. At the first split make a left, then bear to the right down the ramp. Make a left heading east at the four way intersection and you'll see see two cells with gated doors. Use your Large Key on the door to the right (Resistance 82). Inside the cell you'll find a chest. Use your Worn Key to open the chest. You'll never guess what's inside the chest.

That's right ...a KEY!

The next key you'll need is the Silver Key, found in the lower level of the West Wing. You'll be using the Simple Key from the prison cell to gain access to the lower level of the dungeon where the Cuirass resides. Head back to the 4 way intersection and this time go west until you reach a locked door (Resistance 158). Use the Simple Key to open it. Follow the ramps down until you reach the large open room with the greenish glow, full of Shamans, a Ghost Wisp and a Mosswart Chief! Kill the Chief for the Silver Key.

The Silver Key opens the chest containing the Green Mire Cuirass. When it's your turn, be careful! The chest itself is trapped and will cast some Level IV icky spells on you. In addition to the Green Mire Yoroi Cuirass, the chest also contains a nice looking (totally useless) crown.

Note: If you have lockpick, you can go directly to the armor and Yari.

There's a  6th Key, simply described as Key, that can also be found in the West wing. To reach the key, make a right at the split from the entry room, then left up the ramp. At the 4 way split, keep to the right until you reach a room with Shamans and Mudlurks. The key is on a Mudlurk. The chest it used to open, however, (the far end of the west wing) is now unlocked, so the key is simply an artifact of the old quest, prior to its update during the Mirror, Mirror event.