Green’s Dictionary of Slang

(Irish) cow dung.

1945		R. Service Ploughman of the Moon 175: There we were with a wagon heaped high with cow-clabber [...] It was relatively clean and honest cow clap, but the way he scrabbled into it was quite repulsive.
1951		S.H. Bell December Bride 258: He set down his glass and shook his head appreciatively at his brother and Petie. ‘That clears the cow-clap out o’ your throat, all right.’.