This achievement is for speaking to (pressing 'up' while standing over) all inhabitants Iripur. There are a total of 16 inhabitants.
Keep in mind that you will not be able to speak to everyone on your first trip to the town. In order to reach the last handful of inhabitants, you will need the Down Strike ability. This is obtained after defeating the second boss. Once you get the ability, you can use the next teleport you see to return to Iripur and speak to everyone else.
Also keep in mind that some of the inhabitants are children and may be difficult to see at first. You may need to cut some of the grass in order to see them.
It has been mentioned that the achievement may be missable if waited until near the end of the game to attempt. It seems that a few of the inhabitants may vanish from the village. To avoid potentially missing the achievement, teleport back right after you get the Down Strike ability.