Green was on every street corner in South Boston Sunday, as thousands flocked to the annual Saint Patrick’s Day Parade. The sidewalks were full of revelers well before the festivities were set to start at 1 p.m. And with sunny skies and temperatures nosing toward 70, spirits were high. Mike Place, who drove overnight from a similar celebration in Virginia so he could sell green T-shirts, plastic vuvuzela horns, and reams of plastic beads. “I absolutely love it,” said Place, 34, his eyes hidden behind a pair of shining shamrock-shaped sunglasses.