I've long wondered by the old, rusted metal rings attached to the remnants of an old red-brick wall at the back of tiny Mother Cabrini Park, at the corner of President and Van Brunt Streets. I really don't know what their origins are, but I have a theory.
The park was built on the site of an early Red Hook Catholic Church, Sacred Hearts of Jesus and Mary. It started in 1882 at Warren and Hicks. To be closer to the Italian community, it moved in 1885 here where Cabrini Park now sits. The church was a small, flat, three-story building with a gabled roof and one big arched window. It was here that Mother Cabrini, who became America's first saint, worked. The church later moved to Degraw and Hicks Streets, but the older building remained on President Street until the 1970s.
Since the church was built in the 1880s, I'm guessing the iron rings were used to tie your horse to, since that was a major mode of transportation back then.