Do irish people have to wear green on st patrick day?

No Originally the colour associated with Saint Patrick was blue. However, over the years the colour green and its association with Saint Patrick's day grew.Green ribbons and shamrocks were worn in celebration of St Patrick's Day as early as the 17th century. He is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pre-Christian Irish.
This is a short desription on It. Also you will probaly be pinched if you dont wear green though.

you would better.