The term 'Black 47' refers to the summer of 1847, when the Irish famine was at its height and hundreds of thousands of people died from starvation.
The knife Martin Feeney uses is a kukri/khukuri. Originally from the Indian subcontinent, it is mostly associated with the Nepali speaking Gurkhas of Nepal and India. First seen by the British during the Anglo-Nepalese War or Gurkha War of 1814 - 1816.
1 million people perished and died during the Irish famine
At the end of the film, before the end credits, there is the following dedication: "in memory of all those who died, and those who went away, never to return."