I'm sorry OP, it does sound really tough.
In terms of selling their own house, I hate to say it but actually it was probably a really bad move. It is much, much harder to evict someone from a family home in the Irish courts than in the UK - its almost impossible. For them to have been at a real risk of eviction, it would suggest they had paid nothing towards the mortgage and not engaged with the bank for about a decade. The could have negotiated a peppercorn repayment and even if they ran down the equity, would have had somewhere secure to live.
The fact they spent equity that was meant to secure their future housing needs on cars and other upgrades is pretty shocking decision making. And the fact they live with SIL but somehow don't know about the deal she's made, but you do, is a real issue.
I think at this stage, the first thing you need to do is understand if they actually want your help. They may just want a moan, but be unwilling to do anything. If they do want your help, then realistically you want full financial oversight: of their income, their capital, what they owe/own in terms of the cars, what promises SIL may have made them. That should be enough for you to get some independent advise on what they could be eligible for. There is a massive housing crisis in Ireland atm with a huge undersupply of houses to rent. Private renting is not a great option for older people long term, it really is an issue. However there are payments made to 'top up' what people can afford to pay private renting so it is worth investigating. Ultimately though pensions are much more generous than the UK. The state pension is 253.30 a week. That means if they have two full state pensions they have a joint annual income of 26k/year, not including FIL's private pension. Add in a 'decent' private pension and they really should be able to live on that income. So the question is: what support does SIL demand/need? What benefits is she on? How much is she supporting your niece?
If there's a deal that says they invested in her house so they live their rent free, that should be honoured. Otherwise you should look into what has been agreed, and they should understand her outgoings better.
You may find that actually, all these people have enough to cover their basic needs, but are prioritising wants - new cars, holidays. If SIL did a deal on the house with a debt charity, her monthly repayments should have been calculated on what is affordable for her to pay, on her own, based on her benefits. The fact she can't manage this and is demanding pension money from your parents suggests she's overspending in other areas - or her situation has changed since striking the deal and she needs to see if she can make changes. There's a lot that needs unpicking, but I think the narrative you're getting might be significantly underselling what choices people could make now to get out from the situation they're in.