I live in Alaska and by the time March rolls around, we're sick on winter. I'm not doing a St. Patrick's Day board, but instead I found a nice one for "Think Warm" Read about tropical places. Along with it, I'll make palm trees, a big sun (with sunglasses, of course) someone in a beach chair reading a book, etc. Hope this helps.
I do a St. Pat's activity called Leprechaun Limericks. I model limericks for the kids and have them write their own and then I make a bulletin board using a pot of gold and Leprechauns. The kids think this is one FUN activity.
Another one is I trace a huge moving truck onto the bulletin board and title it Moving Into Spring. We do a writing assignment about how winter moves out and spring moves in! Good Luck.