Members of the Association of Secondary Teachers, Ireland (ASTI) also backed industrial action over lower pay rates in place for more recent entrants to the teaching profession “to be taken in conjunction with one or both of the other teacher unions”.

Was this original approach not agreed by this very union? i.e pull the ladder up after themselves once they got everyone a cushy payoff and fuck all the millenials?

It's one of the many moves by companies/governments to slowly ebb away unions power. It's quite clever because i'm not joining my company Union for the very same reason. I get paid less than my colleagues because of the established agreements before i was even employed.