First, "Social justice" is a broad and very diverse movement of people wanting to reduce the amount of (real or perceived) injustice people face for a variety of reasons (skin color, gender, sexual orientation, place of birth, economical position, disability, ...). Like in any such broad political movement, subparts of the movement are less rational than others.

Overall, "social justice" is still mostly a force of reason and rationality against the most frequent and pervasive forms of irrationality in society, which are mostly religion-based, but yes it varies from subparts of the movement. It is, historically, a byproduct of the Enlightenment after all.

That said, there are several levels of "rationality" and "rationalism", and it might be very rational to make irrational demands.

When you make demands in social and political context, you know your demands will usually not be completely fulfilled. Asking for something "impossible" may be the best way, from a game theoretical point of view, to end up with having something not too far from what you really want - the same way that when you're bargaining the price of an item in an informal market (like in latam or maghreb).

It can also be a powerful way to make people think about a question in novel ways and try to find alternative solutions which aren't part of the hypothesis space they usually wander. "Abolish prisons" may seem an irrational demand, and it's very likely that something "like prison" will be required for a few very dangerous individuals, but it can make people think about possible alternatives to prison, something they don't usually do, and which could very well be used for 90% or even perhaps 99% of people currently in prison.

Of course, making "irrational" demands can also be counterproductive, it can discredit the movement, may you appear to be a lunatic, ... but it's a powerful tool to have in your toolbox when you rationally pursue some deep changes in society.