Green’s Dictionary of Slang

slat mara n.

[Ir. slad-mhara, murder and robbery; slad-mharoir, freebooter]

(Irish) a contemptible person.

1927

P. O’Donnell Islanders (1933) 100: Arrah, damn on ye! it’s not the nerve to hoult a girl ye’d have herself, ye poor slat mara.