Keep a paper trail of you paying her fees. Any money you give her, pay into her account from yours. Anything to show 100% proof that she is an adult who is dependant on you.
If she was younger then you wouldn't need this proof as it accepted that a child is dependant on their parent. As she over age 21, you must prove that she is an adult who is dependant on you.
The EU service said that if I come in as a worker OR as a retiree, she would be free to seek work (outside of schooling). Do you have information otherwise?
I don't know why they told you that. You can't come in as a worker if your aren't one.
There isn't a retiree qualified person, it would be a Self Sufficient qualified person. A Self Sufficient qualified person needs proof they have the funds/income to be self sufficient. They and all their family members must have a a CSI (Comprehensive Sickness Insurance) or an S1 from their own EEA country as that is used as a CSI because their own country pays all their NHS bills. No taking UK benefits as they are self sufficient.
If you were a self sufficient qualified person, UK laws do not treat an Irish citizen as "foreign". I am
guessing that this would mean that you residing in the UK could use the NHS for free, but your daughter would need a CSI as you are using EU rules for her? TBH, I've only ever seen Irish worker qualified persons using the EU route in the UK for their non-EU family. A worker qualified person and all their family members, can use the NHS for free.
An EEA citizen is allowed a three months "visit" in another EEA country, using our own countries EHIC to pay for any healthcare we need in there or we use our own private insurance policy. After that 3 months, we have to leave if we aren't a qualified person by then.
Being a "qualified person" at all times is our "right to reside" in that country with our family members. Cease being a "qualified person", then we and our family members lose the "right to reside" in that country.
Unlike the UK "visa" that gives "limited leave to remain" with an end date, EU treaty rights is about EEA citizens and our families having a "right to reside" in that country, which ends as soon as we (the EEA citizen) cease to be a "qualified person".
As for her working, yes if you remain a qualified person
at all times, then she is allowed to work. The UK has just made working when not allowed, a crime. It is important that you remain a qualified person at all times so that she continues to have a "right to reside" in the UK.
However, you are claiming that she is an adult who is dependant on you and her 'right to reside' in the UK as your family member, depends on that.
I'm not sure how much leeway the UK will give on that if she is working to keep herself. You need a solid paper trail at all times, to show that you always keep her.