A light blue became associated with the Order of St. Patrick.
The earliest use of green for nationalistic reasons was seen during the violent Great Irish Rebellion of 1641.
The Society of United Irishmen, wore green, especially an Irish version of the "liberty caps" worn during the French Revolution.
The importance of the color green spread, thanks in part to the poems and ballads written during this time, most famously "The Wearing of the Green."
The origins of the wearing of green clothing in the U.S. on St. Patrick's Day and for St. Patrick's Day celebrations in general date back to the 19th century, when waves of Irish immigrants came to America looking for better job opportunities, especially after the Great Famine of the 1840s-50s, and began wearing green and carrying Irish flags along with American flags as a point of pride for their home country.
http://time.com/4699771/green-irish-st-patricks-day-color/?xid=newsletter-brief
