Impolite and indecorous to leave Northern Ireland in awful outfit (12)

I believe the answer is:

discourteous

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'impolite' is the definition.
(similar in meaning)

I cannot understand how the remainder of the clue works.

(Other definitions for discourteous that I've seen before include "Impolite" , "Rude, inconsiderate" , "Ill-mannered" , "Uncouth" .)

I've seen this clue in The Irish Times.

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