Ryanair has been operating Bristol-Knock for ten years, mainly as a summer route (3/4weekly) although it began as year-round. Interestingly, it will operate through the coming winter for the first time for many years at 3 x weekly. The early summer load factors on BRS-NOC have been similar to that on BRS-SNN for the first month of that route (part month to be precise) this year. For example, in May of this year BRS-NOC load factor was just under 73%, similar in 2017, 69% in 2016 and 72% in 2015. So the new BRS-SNN route does not appear to have impacted negatively on BRS-NOC, albeit less than a month's operation is a very small sample of course.
The BRS-NOC load factors typically improve significantly as the summer progresses each year so hopefully the same will apply to BRS-SNN. As always, yields are the important figure.