One of the mysteries of the English language finally explained.
Definition of Oirish in English:
Oirish
noun
derogatoryFrequently with the: Irish people considered collectively.
adjective
derogatoryOriginally especially in representations of Irish speech: = "Irish". Later also: exaggeratedly, sentimentally, or putatively Irish in style, character, etc.
Origin
Mid 19th century; earliest use found in The Caledonian Mercury. Representing an Irish English pronunciation of Irish and Irish, often taken as indicative of exaggerated or pseudo-Irish speech.