The fireplace may be bricked up for a reason. The chimney may have problems that you can't see from the outside. Have a chimney service check it out. You can probably remove the bricks yourself to expose the firebox, but if the chimney is not safe you would be wasting your time. A crack inside the chimney could cause a fire. It may just be that the people did not use that fireplace and wanted to eliminate the cold air into the room. But have it inspected to be sure.