Landlord will only accept cash

You must have missed the memo. People are always guilty until proven innocent these days. You can say anything about them and the are automatically guilty.

The OP merely needs to tell Revenue what she is paying and mention there are three of them.

Revenue can easily cross reference it with the landlords returns to confirm that the landlord is most definitely tax compliant. So there will be no need for an audit of course.

So what would be your threshold for thinking someone not declaring income went from good to bad?

If we take the OPs story at face value, and assuming that the landlord is not declaring, that is a sideline income of 40k. What is the point at which your "disdain" would vanish? 100k? 1 million? 10 million?

(BTW, Revenue would likely not bother auditing if the landlord is declaring the income. A 5 min check on their returns would confirm that)

Ah here, that's a complete cop out. What are you going to do - ring them up and say "hey, I was thinking of reporting you to Revenue, but I just wanted to make sure you don't know anyone who has bone cancer first."

How about the self-employed millionaire who you know is committing millions in VAT fraud importing garlic? Shure ya don't know his circumstances. The poor divil might be addicted to coke and hookers. Ya better not dob him in just in case

Lots of people paying these types of rents to share with others.

OP, I would report that. In this day and age its not acceptable that people want top dollar for rent and not want to pay tax. €14k tax free is a serious benefit, yet they want more...

I wonder if its a possibility that they are not the owner and are paying another landlord, possibly a friend or relative in cash, who in turn are not declaring it.

No harm to let revenue have a bit of a look around this.

Before anyone, asks I am tax compliant, and will not do business with anyone in cash.

40k a year is minimal? Well over 13k x 3 according to the OP and assuming all three tenants are on the same rate

What is your threshold be for something not to be minimal?

You have an interesting view in tax. Why would you pay other people's tax?? That seems a daft thing to do.

I agree with you being audited is an inconvenience. The easiest way to put money towards avoiding this is to... simply pay the tax you owe.

If the landlord is is tax compliant it'll be straightforward to prove. There should be nothing wrong with holding people to account when the rules are there for all.