Manxnotebook has considerable bits on place names but I don't recognise the major component of the name (not obvious in my Manx dictionary)
Cronk is just manx for hill (= cnoc,knoc, knock etc as in Irish + also several manx placenames) na = 'of the' but what Myrrhon is I don't know.
The place name could only have come from either the register or will so they are the first place to check - both on mormon microfilm.