Green milkshakes served to celebrate St. Patrick's Day, especially at McDonald's.
Also could be a reference to the large Irish influence and population in Boston. For example:
- People of Irish descent form the largest single ethnic group in the city, making up 15.8% of the population. 
- Irish Americans are a major influence on Boston's politics and religious institutions. 
- Irish immigrants dominated the first wave of newcomers [to Boston] during [the 1820's] period. By 1850, about 35,000 Irish lived in Boston.
from For Boston