A common expression in Ireland meaning the poorest of the poor - a person whose trade is selling cow-dung as a manure.
In Cork the inhabitants of the poorer districts were defined as 'shitehawks who came in from beyond the lights years ago with muck between their toes and nothing between their ears'; 'Beyond the lights' meaning outside the city limits or the area covered by street lighting. Manure selling was actually a trade in Cork. People took handcarts into the countryside, collected cowdung and hawked it to gardeners as a cheap fertiliser.
If you value your life never refer to an Irish person as a 'shitehawk'. He or she may be quite offended.