The reason why an outbreak of the potato blight Phytophtora Infestans resulted in a large proportion of the population of Ireland being without food is, of course, because an overwhelmingly large proportion of the Irish population relied solely on the potato as their source of calories. The relevant historical question is why such a reliance came about. this great answer by u/ParkSungJun explains how the privatisation of land into the hands of landlords in the preceding centuries shunted people into tiny rented plots where the only crop that gave the necessary calories per square metre cultivated was the potato. Plenty of other crops were grown in Ireland, but of course people did not own them and had no money to buy them (The reason your question has been downvoted is because its written as if you think Irish tenant farmers had a choice in the matter and their potato reliance was their fault).