passive voice - Parsing potential phrase modifying a noun

From Hirameki Hatsume-chan Vol 1, chapter 71 How does the sentence in the third panel translate, and who is doing what to whom? Is it a potential and the どうぐ is "enabling" him to have good dreams or is it passive and the 見る verb used to mean "show"?

Answer

［（私が/人が）いい夢を見られる］道具

A tool [with which (I/people) can see a good dream]

いい夢を見られる is a relative clause modifying 道具.
Its non-relative equivalent would be:

道具で（私が/人が）いい夢を見られる。
With a tool, (I/people) can see a good dream.

So the 見られる is potential. Its subject can be "I", "we", "you" or "people" who will have a good dream using that tool. The particle で goes missing when you turn the sentence into the relative clause.

Compare:

• 
  

  私がペンを使った。-- I used the pen.
  → ［私が使った］ペン -- the pen that I used (を goes missing)

  
  


• 
  

  道具で火を起こす。 -- (I) start a fire with a tool.
  → ［火を起こす］道具 -- a tool with which (I) start a fire (で goes missing)

  
  



• 
  

  筆できれいな字が書ける。 -- (I) can write beautiful characters with a brush.
  → ［きれいな字が書ける］筆 -- the brush with which (I) can write beautiful characters. (で goes missing)