How to cut in paint on a TALL wall left of a stair case
Around stairs you either learn to work with pole-attached tools (corner foam wheel might do what you need) or you will be renting or purchasing some elaborate staging, or you will be finding that the elaborate staging would have been a lot cheaper and much less trouble than the trip to the emergency room or morgue. 

Admittedly the latter isn't much trouble for you, personally, at that point...

If it's hard to reach it's also hard for nit-pickers to examine closely and whine about the quality of the paint job up there.