What cause so many people to leave ireland in the 18th century

Many local people began their long emigration trail during the 1800s, being rowed out to catch a passing schooner bound for Glasgow or Londonderry where they would embark on one of the many emigrant ships to Australia, New Zealand or the Americas.

If the 17th and 18th century Penal laws of the Royal Crown leveled at mostly Catholic society could be summed up in one word, the word "brutalisation" just might be the more accurate one to employ for those times. From at least as early as the year 1603, laws then enacted, seemed to focus on their society perhaps as much as any non-parochial one in the whole realm. For example, imagine a family homestead which prior to this time was once held by the family for several centuries, but was suddenly ripped from beneath their feet and which forced many onto the 'street' in abject poverty practically overnight.

These and other intolerable conditions in Ireland forced Irish (especially Catholic) emigrants to leave the country.

Here is a view of four core reasons that motivated or forced our Irish ancestry to turn their backs on their homeland, in order to thrive in a new existence abroad

Explanation:

The Potato Blight

Explanation:

The Potato Blight which destroyed the staple of the Irish diet produced famine. Hundreds of thousands of peasants were driven from their cottages and forced to emigrate -- most often to North America.

you answered your own : -/