Company trying to force employees to sign NDA for failed payments

So as the title says my company is trying to get employees to sign NDA's so we can't discuss our pay with anyone. This is stemming from a WRC case I had and the result was I needed to be paid for Sunday Premiums that they refused to pay for. I got paid in full and that was great and that also meant they needed to pay the rest of my Sunday co-workers. The company is lowballing what they owe my colleagues (by over 90%) and telling them if they discuss it they don't have to pay them. My question is: Is this illegal for the company to ask/do?

This is more of a passive question (curiosity) as the co-workers wouldn't go to the WRC so they're having BS flung at them all the time (I've told them to go to the WRC multiple times). Would this NDA even be legally binding considering it's pay related and is 100% legal to disuss? I personally think they're pulling all of this because my co-workers are not Irish and the company is trying to pull the wool over their eyes. I also think it's to insight fear and make them not speak to anyone about it and accept a monstrously low figure.

Again this is more from curiosity as I'm blue in the face from telling them to go to the WRC or a solicitor.