I got grinds in college on a subject i'd never done in school (95% of course had done and lecturer seemed to assume this), and found it quite intense, having 1 or 1 tutoring, again maybe that's just me.
It's definitely not just you, but it is a somewhat different scenario - you had effectively no grounding in the subject, whereas allthedoyles's daughter probably knows more than she even realises herself. A few hours of extra support over some weeks / months should (hopefully) have the effect of bringing that to the fore.
And although junior cert isn't hugely complicated maths, many parents will have gone through a completely different syllabus, may have rusty skills - and may well value an independent external person who the student may listen to more (and may not be susceptible to the same level of frustration!).
I also think that maths is one of the worst subjects to give up on at a very early stage, because it's a requirement at least to have ordinary level maths for so many third level courses and in any case it's necessary at some point in many jobs. For life in general most people don't need much more than arithmetic on a day to day basis, but it's often useful to have more. Bar possibly matrices. I've never found the application of matrices to have any point at all in normal life. Algebra, yes (surprisingly often); geometry, yes; calculus (I'm a sucker for punishment), yes. Even logs. Matrices? Tchah.
Granted, not everyone is good at anything, but I'd provide positive intervention, give it some time to take effect, and only then make the assessment as to whether the student will be able for it. To paraphrase
very pompously, if at first you don't succeed, give it at least another attempt and some effort before downgrading expectations.