Ruby, 69 bytes

->s{"#{(s.capitalize!||s)[-1]==?y?s:s+?y} Mc#{s.gsub /e?y$/,""}face"}

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Ruby, 69 bytes

->s{"#{(s.capitalize!||s)[-1]==?y?s:s+?y} Mc#{s.gsub /e?y$/,""}face"}

Explanation:

->s{                                                                } # lambda 
    "#{                                 } Mc#{                }face" # string interpolation
       (s.capitalize!||s) # returns string capitalized or nil, in that case just use the original string
                         [-1]==?y # if the last character == character literal for y
                                 ?s:s+?y # then s, else s + "y"
                                              s.gsub /e?y$/,"" # global substitute
                                                               # remove "ey" from end

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Ruby, 69 bytes

->s{"#{(s.capitalize!||s)[-1]==?y?s:s+?y} Mc#{s.gsub /e?y$/,""}face"}

Ruby, 69 bytes

->s{"#{(s.capitalize!||s)[-1]==?y?s:s+?y} Mc#{s.gsub /e?y$/,""}face"}

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