Slievenisky can be reached easily enough from the access for Slieve Croob such as the amenity parking at J30015 45240 A. The land between the amenity area and the summit is rough, sometimes boggy and tricky navigationally in mist but walkable for the experienced. Of the two spurs of Slieve Croob this top is the wilder. It has good views of the coast and the northern Mournes.
The top has two rivals for the highest ground. As of 2013 MV is showing the highest point of the larger area shown on the OS map as 446m high. Linkback: https://mountainviews.ie/summit/631/comment/5390/

I tackled Slievenisky form Slieve Croob, which is only 1.2km away and there's a handy service road up from the car park. There's a handy fence that makes navigation straightforward, even in low visibility. Climb over the fence and follow it SE to near the summit of Slievenisky, before turning right near the top. The summit area is on a prominent bump; you can't miss it. Again, low cloud limited the views. Linkback: https://mountainviews.ie/summit/631/comment/3206/

Done this in a loop with slieve croob, slivenagarren, and cratlieve. Clear day and it is not a problem to do a loop. We parked at slieve croob car park Linkback: https://mountainviews.ie/summit/631/comment/21506/

Followed the paved road up Slieve Croob and from there the narrow track (located very close to the trig pillar) SE that leads most of the way to Slievenisky's summit, which is a slightly elevated area of peat with good views looking down into the valley to the west. Linkback: https://mountainviews.ie/summit/631/comment/23212/

Like Cratlieve, the other spur off Slieve Croob, this peak seems to only be walked by those who have taken in it's bigger neighbour first. And the numbers suggest that only about a quarter of MVers do this. For those who just want to take in this summit, for whatever reason, here it is. There is a wooden gate at J3190543624 B just east of a small river. Enter the field and follow farmers track parallel to the river until you reach another gate at J3190243897 C. Go through the gate, there is a fence to your right which you can follow to the summit.

As you can see from the attached photo I walked this hill during the recent snow. My approach was from Slieve Croob and used the above route as the most direct route of getting off the open mountainside and on to a road (as I was in a rush to get back to Castlewellan in time to see the Grand National!). Not the conditions to really judge it, but I would wager that the terrain under foot is better from this approach than the boggy saddle between Slieve Croob. So if you have already done Croob and have decided to return to the area to also tick this and Cratlieve off your list I would highly recommend this route.

On an aside, isn't it amazing how a fall of snow can make what would not be the most exciting of hill walks into a greater adventure. On both Cratlieve and the small Grungadoo I came across five feet frozen drifts that allowed me to walk over the gates. Definitely a lot of fun to be had in the snow. The only regret I had was that I hadn't brought a small sledge for the descent, particularly on my visit to Doan last weekend! I do love the Mournes and the recent snow has only made me love them more. Linkback: https://mountainviews.ie/summit/631/comment/14972/