SAVANNAH, Ga. — There was no green beer, no pickup trucks towing parade floats or throngs of tourists in gaudy green outfits. Instead, just a few dozen of Savannah’s early Irish immigrants marched a half-mile from a hotel near the riverfront to attend a special service at what then was the city’s only Roman Catholic church.That modest procession on March 17, 1824, launched one of the most beloved — and profitable — traditions in Savannah. On Saturday, Georgia’s oldest city will…