We were at EPCOT 2 yrs ago on St Patricks Day, there was nothing special going on, but we sure had a great time explaining to some of the CM's around WS that didn't know what St Patricks Day was. (We had on our Old Navy St Patty day shirts) We were also asked to be the Family of the Day, but just as we were being asked, another CM stepped up and apologized as she had just asked another family. Don't know about the other parks.
We were in the parks for St Patricks day about 6 years ago. Nothing done for the holiday. We were there for my little sister's bday, thre were 12 or so of us and we all wore our green derby hats... actually got some weird looks, lol.