This is confusing.
Can a PAYE worker who has been forced to work from home for 3 months get money back from revenue, if their employer does not pay them any extra for working from home?
If so how do they go about getting this money?
I think that is the situation 90% of the workforce are going to be in.
It really isn’t confusing. Stop and forget whatever you have seen that has confused you. It may be a bit involved, but there's nothing confusing about it.
Your employer can choose to pay you a tax free amount of UP TO €3.20 per day, and it's well established that this would be by far the most attractive thing financially for an employee.
If your employer doesn't pay you any allowance for working from home, then you just have to forget the existence of the €3.20 a day thing - it's only relevant if you're getting it!
In that situation you have to make a claim for a tax relief after the end of the year - you'll be able to do it via MyAccount - you probably can't do it for 2019, but as you said there will be hundreds of thousands of people looking to make this claim next year, so you can be pretty sure that Revenue , who have no interest in making a cross for their own backs, will have a systems development for the online service, to allow people to claim, in the same way as they do for medical expenses etc...
What you can claim for is not complicated at all, it's common sense really. The additional cost of utilities etc, due to your direct usage of them in the course of performing your work at home.
It won't be a life altering sum of money, as for a month period it will be 92/365 (or about 25%) of 10% (or whatever the appropriate % of your property is a fair % to attribute to your workspace) of your light & heat bills.
Lets say you're an optimist and reckon your workspace accounts for 20% of your gaff. This means you'd be claiming 5% (i.e. 92/365 of 20%) of your light & heat bills for the year. That is deducted from your taxable income for the year, meaning you get tax relief at your highest rate of tax. Depending on what tax band you are in, that would equate to cash in your hand of either 20% or 40%, of 5% of your utilities for the year. If your utilities are €2,500, that's €25 - €50. And that's based on claiming your home office takes up 20% of your home, which is twice the 10% that Revenue have indicated they reckon is a reasonable estimate.