Irish English: What is cat melodeon?

Don't shoot me I'm only the piano accordion player!

Cat/Cat melodeon (a): dreadful, no good, awful, very bad. 

Bernard Share’s dictionary of Irish slang Slanguage quotes Victoria White in the Irish Times calling cat melodeon “the greatest expression in Hiberno-English.” 

The word cat is used to express disappointment in the quality of something: the food is cat in that place.

Where does Cat Melodeon come from?

The Cat Melodeon players

In his book on Irish traditional music, Ciaran Carson suggests cat melodion is a joking reference to the musicianship: of piano-accordion players (who often refer to their instruments as melodeons) to play two notes at once. Source   

As the nephew of a fine melodeon player, I think this is cat altogether  -  you throw in altogether for emphasis, by the way.

When was it first used?

Strangely, some dictionaries cite the first use in print as being in the 1980s. This is decades after I first heard it. My guess is that it has been common in spoken hiberno-English for at least a century.

How is it used?
Cat is a lexical Swiss army knife but usually expresses displeasure.  I have most often heard it as a yelp of protest: That film was cat altogether!' 

Further readingA Foreign Country - photo of family gathering reveals a poignant story of Irish exile in the 20th Century.