It is useful to compare designs with contemporary buildings. This picture is from a nearly 1830's property. Although the staircase in 19 Abercromby square wasn't put in until around 1890 ( apart from the post at the bottom), it very closely closely matches the look of the earlier design.
While we can only surmise at this stage, the original 1863 stair rods and bannister were presumably of a very similar design and look. The other ground floor bannister and stairs to the billiard room in the basement looked quite different though, less ornate and closer to a 'southern' style.