How difficult is it to install an iron banister in this location?

How do you define “Banister”? If you’re referring to the whole thing, reach out to your nearby iron fabrication shop as they are experienced with these. These are custom-made and installed based on the job requirements. If you’re looking for a wooden handrail, newel, and bottom plate with iron spindles, it requires a certain level of expertise. If you feel unsure, it’s best to contact a skilled trim carpenter for assistance. It’s not a massive project, but skills are necessary.