Lots of noise about "Look at Sinn Fein! The Republicans are coming!"
So I grabbed the numbers from Wikipedia all the way back to the first NI Assembly vote in 1998, and it's quite clear that the numbers for Republicanism in general have hardly nudged in the last 25 years. SF+SDLP goes back and forth over the 40% line by a point or two.
The real story is that Unionists are slowly moving to Alliance. It's taking a generation or three, and past performance is not a guarantee of future performance, but if you look at where the figures have gone, that's why SF has moved into first place - not because Republicanism has taken mindshare, but because enough people have decided that Unionism isnt for them.

You'll have to excuse the fairly constant 6-7% of "Unknown" at the bottom. I wasn't going to go through every tiny party from the last 25 years and work out whether their 0.7% of the vote was Republican, Unionist, or Other.
For the record - Republican is SF and SDLP, Unionist is DUP,UUP, PUP, and TUV, and Other is Alliance, Green, and People Before Profit (who I'm fairly sure are Other, but am happy to be told otherwise, although it won't make much difference to the graph shape.