Mmm, that sounds strange.
Obviously the licenses premises can't assume that just because the person is between the ages of 18 and 23 they are a possible/potential risk so that cannot be an acceptable justification for this policy. Actually, I'm pretty sure they have to substantiate why a person is a possible/potential risk (i.e. they can't just say 'He looked like he was trouble' - they need to prove that the person was either acting threateningly or drunk).
The Equality Status Act includes age as one of the grounds for discrimination - e.g. you cannot discriminate against someone based on age, just the same as you cannot discriminate based on race.
However, I think there may well have been some recent revision to the legislation, which is probably what CMCR heard, as I can't imagine the pubs would come out with what would otherwise amount to blatant breaches of it.
By the way, a pub or a bar, are both public house and, as such, are public, not private, premises, and so the management do not reserve the right to refuse admission, except, when the person is not of legal age, or is drunk or posing a threat (and some other clauses, but the pub does not have the right to refuse admission because they do not like the look of someone, or the person is not a regular).
On your point on the admittance of the older person to the cafe: so what the Equality Authority essentially told you was that the fact the person was old should be irrelevant to deciding if they should be granted admission?