Green’s Dictionary of Slang

(Irish) extremely (drunk).

1952		B. MacMahon Children of the Rainbow 70: Twice, as he was on the point of pitching us to hell, we made him blithero drunk.
1974		Star Press (Muncie, IN) 24 Oct. 23/2: The Irish have at least two dozen words for inebriation [...] killarneyed, fluthered, stotious, pallatic, maggoty, blithero, half-tore, paralytic and stoven.