A decision under Article 26 that legislation is constitutional means that the constitutionality of that specific piece of legislation cannot be considered again.
As to whether the Supreme Court is bound by precedent set by previous Article 26 decisions (their Ratio decidendi) is more open to question.
In Re The Illegal Immigrants (Trafficking) Bill 1999  2 IR 360, the Supreme Court were invited to depart from previous decisions that bills submitted to them under Article 26 had a presumption of constitutionality. They declined to do so, saying:
The court is, accordingly, being asked in the case of the present
reference to overrule two earlier decisions of the court where
precisely the same issue was debated and resolved. It could be said,
by way of comment on the arguments advanced by counsel assigned by the
court on this issue, that they give entirely insufficient weight to
the fact that the President, although doubtless an integral part of
the Oireachtas, plays no part whatever in the purely legislative
function of the Oireachtas. It is, however, sufficient to say that,
whatever the merits be of the arguments advanced in this issue, it has
not been demonstrated that in The Criminal Law (Jurisdiction) Bill
1975  IR 129 and The Matrimonial Homes Bill 1993  1 IR
305, any relevant argument or authority was overlooked in the
judgments of the court. Nor are there any other compelling reasons
which would permit the court to depart from those decisions. The court
is satisfied that this does not constitute an exceptional case as
defined in the judgment of Kingsmill Moore J. in The Attorney General
v Ryan's Car Hire Ltd  IR 642. It follows that the court will
approach its decision on the reference on the basis that the
presumption of constitutionality applies to the two provisions under
consideration. (at page 369, emphasis added)
It would follow, therefore, that the court might reconsider a previous decision it made under Article 16, if they felt there were "compelling reasons" to do so, which constituted "an exceptional case".
Indeed, the last the of the decisions cited by the court, Ryan's Car Hire, was not an Article 26 reference, but was a seminal decision that established the Supreme Court could overturn its old decisions (and those of its predecessors). Accordingly it's arguable that the reasons from departing from an Article 26 decision would be much the same as departing from a decision in any other case.