The pixies won't let you go free until you have completed some chores for them, and Oul Sigun has one such task.
This quest is available if you have completed the quest Il Mheg, the Faerie Kingdom.
Starting the Quest
Talk to Oul Sigun in Il Mheg to begin the quest.
Draw Water from Longmirror Lake
Go northeast to where the orange arrow is pointing in the minimap and check on the sparkling Destination. Defeat the two Fuath Pranksters that attack.
Give the Water to the Leafmen
Go southwest to one of the quest icons on the map. Dismount and target the Leafman, and then you can either examine or target it and use the Longmirror Water on it from the Key Items section of your inventory, or you can target it and use the icon to the right of the quest name in your list of active quests.
Then go to the other quest icon on the map and do the same.
Speak with Oul Sigun
Talk to Oul Sigun to complete the quest.