Green’s Dictionary of Slang

(Irish) a fool, a useless object.

1883		T.B. Reed Willoughby Captains (1887) : ‘Boo hoo! Who got kicked out of his boat! Young muller, couldn’t steer a tub’.
1920		Ade Hand-made Fables 253: If she read a Novel containing a Love Scene, the other Maud Mullers thought she was a bit Daring.
1952		B. MacMahon Children of the Rainbow 75: An awkward old muller that same boat was! After a struggle we succeeded in getting her to ride the almost silent tide.
1976		T. Murphy On the Outside 12: And she’ll say, ‘Who did you come with?’ And you’ll say – oh, Mickey Ford or someone [...] What’s all the fuss about this one for anyway, she’s only a mul .