What did the Trent lack with regard to Immaculate Conception that Ineffabilis Deus did not? I've often heard it alleged that 'the Immaculate Conception wasn't a dogma until Pius IX defined it in 1854.' However, I can't reconcile this with what the dogmatic Council of Trent already declared 300 years prior.

Namely,


  
  If any one does not confess that the first man, Adam [lost original righeousness]  ... ... let him be anathema.
  If any one asserts, that the prevarication of Adam injured himself alone, and not his posterity ... ... let him be anathema.
  If any one asserts, that this sin of Adam,—which in its origin is one, and being transfused into all by propogation, not by imitation, is in each one as his own,— is taken away either by the powers of human nature, or by any other remedy than the merit of the one mediator, our Lord Jesus Christ ... ... let him be anathema.
  If any one denies, that infants, newly born from their mothers' wombs, even though they be sprung from baptized parents, are to be baptized; or says that they are baptized indeed for the remission of sins, but that they derive nothing of original sin from Adam ... ... let him be anathema.
  If any one denies, that, by the grace of our Lord Jesus Christ, which is conferred in baptism, the guilt of original sin is remitted ... ... let him be anathema.
  
  
  This same holy Synod doth nevertheless declare, that it is not its intention to include in this decree, where original sin is treated of, the blessed and immaculate Virgin Mary, the mother of God; but that the constitutions of Pope Sixtus IV., of happy memory, are to be observed, under the pains contained in the said constitutions, which it renews.
  
  —Council of Trent, Session V, First Decree, On Original Sin.





  ...
  
  CANON 23 — If any one saith, that a man once justified can sin no more, nor lose grace, and that therefore he that falls and sins was never truly justified; or, on the other hand, that he is able, during his whole life, to avoid all sins, even those that are venial, —except by a special privilege from God, as the Church holds in regard to the Blessed Virgin; let him be anathema.
  
  ...
  
  —Council of Trent, Session VI, On Justification.


Question

If this was already dogma per Trent's decree, why did Pope Pius IX define it with an extra-ordinary definition?
Pope Pius IX wrote in Ineffabilis Deus explicitly: 


  Besides, we must note a fact of the greatest importance indeed. Even the Council of Trent itself, when it promulgated the dogmatic decree concerning original sin, following the testimonies of the Sacred Scriptures, of the Holy Fathers and of the renowned Council, decreed and defined that all men are born infected by original sin; nevertheless, it solemnly declared that it had no intention of including the blessed and immaculate Virgin Mary, the Mother of God, in this decree and in the general extension of its definition. Indeed, considering the times and circumstances, the Fathers of Trent sufficiently intimated by this declaration that the Blessed Virgin Mary was free from the original stain; and thus they clearly signified that nothing could be reasonably cited from the Sacred Scriptures, from Tradition, or from the authority of the Fathers, which would in any way be opposed to so great a prerogative of the Blessed Virgin.[12]
  
  [...]
  
  
  Sess. V, Can. 6; Denz. n. 792. Declarat tamen haec ipsa sancta Synodus, non esse suae intentionis, comprehendere in hoc decreto, ubi de peccato originali agitur, beatam et immaculatam Virginem Mariam Dei genitricem, sed observandas esse constitutiones felicis recordationis Sixti Papae IV, sub poenis in eis constitutionibus contentis, quas innovat.
  


So he explicitly mentions the first quote in the question (the latin text in the footnote is the last paragraoh of your quote), but that did not make him think he should not write the encyclica.

When you look closely at Trent's Decree you see that they only say that they make no definitions on Virgin Mary. They know the title Immaculata and agree with it, but they do not make an exact definition about this. Historically speaking the Council of Trent wanted to condemn the teachings of the protestants, but did not want to restart (and then end) the discussion about the Immaculate Conception of the Middle Ages.

The Council never said "If any one denies the Immaculate Conception let him be anathema." and also never explained what the concept of Immaculate Conception means in detail. Both did Pope Pius IX. His encyclica displays the Council as if it would have said everything before, because a dogma is a teaching that was ever-believed by the church and he wants to prove this. But he knew that there was some work to do left, so he did it.

Making something to an dogma means (ideally) that a faith the church had through all times is exactly defined. This can for example be needed because of an attack on this faith from inside or outside the church. Pope Pius IX has done exactly this with Ineffabilis Deus.