At present there seem to be two schemes by which holders of UK short-stay visa may enter Ireland: The SSWVP and the BIVS. Given their partial duplicate coverage of the same nationalities (India, China) it would seem reasonable to assume that they are separate legal mechanisms (to my understanding BIVS visa constituting basically combined UK+Ireland visa, while UK short-stay visa under SSVWP just being "accepted for short-stay entry"), but I cannot find any definitive source stating how exactly the two schemes are interrelated.
Hence my question: Are these two schemes directly linked, i.e. amendments to one affecting the other in turn, or are they independent of each other?
Please cite credible sources in answers!
(Note: strongly related to my question on Travel, but in this instance more about the general legal interconnection between the two and not about the current state of effect)