What is the difference between Irresistible Grace and the Immaculate Conception?

If your interpretation of Selective Election is correct¹, the answer is fairly clear per the scriptural passage where Mary says yes (Luke 1:38 -- be it done to me according to thy will) ). Mary had the option to say no, but didn't. The other premise seems to preclude choice so that the option of "yes" or "no" never arises. The Catechism describes the call to faith as an invitation by God. An invitation can be accepted, or declined.

• For what it's worth, Mary's "yes" gets a lot of emphasis in our parish and in our diocese. It's the topic of various homilies and other pastoral encouragement to the laity to align our will with that of God's. As our pastor has put it "you are presented with opportunities every day to 'say yes.'"

• The yes or no choice for Catholics is related to repentance, which represents a turning toward God, or a return to God depending on how you refine the sense of that term -- repentance-- as it comes to us from the original Hebrew and Greek terms.

  The Catechism opens with the call to a relationship with God, through Christ, by God's invitation rather than His fiat.

  I. The life of man - to know and love God

  1 God, infinitely perfect and blessed in himself, in a plan of sheer goodness freely created man to make him share in his own blessed life. For this reason, at every time and in every place, God draws close to man. He calls man to seek him, to know him, to love him with all his strength. He calls together all men, scattered and divided by sin, into the unity of his family, the Church. To accomplish this, when the fullness of time had come, God sent his Son as Redeemer and Saviour. In his Son and through him, he invites men to become, in the Holy Spirit, his adopted children and thus heirs of his blessed life.

  2 So that this call should resound throughout the world, Christ sent forth the apostles he had chosen, commissioning them to proclaim the gospel: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age."4 Strengthened by this mission, the apostles "went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it."5

  3 Those who with God's help have welcomed Christ's call and freely responded to it are urged on by love of Christ to proclaim the Good News everywhere in the world. This treasure, received from the apostles, has been faithfully guarded by their successors. All Christ's faithful are called to hand it on from generation to generation, by professing the faith, by living it in fraternal sharing, and by celebrating it in liturgy and prayer.6

Father James Martin's commentary (he's a Jesuit scholar) is an example of this common theme, Mary's "yes" ...

But the decision is always up to us. We are free to say yes or no to God.

With her yes, Mary partners herself with the Almighty and is empowered to bring Christ into the world. This world-changing yes is what St. Bernard speaks of in one of his sermons on Mary:

• Answer with a word, receive the Word of God. Speak your own word, conceive the divine Word. Breathe a passing word, embrace the eternal Word.

With our own yes to God's voice in our lives we are also asked to nurture the word of God within us and bring Christ into the world—certainly not in the same way that Mary was, but in our own situations, and in our own ways. Using our own talents and graces we are called to bring Christ into the lives of others.

¹ Based on this summary by Rev Bryn MacPhpail, I think that "limited atonement" may be what you mean by Selective Election

The only interpretation of "limited atonement" that is faithful to Calvin's beliefs is that God does not will everyone to be saved. This is not to say, however, that the means to salvation (Christ's expiation of sin resulting in propitiation) is not accessible to all. The command to proclaim the gospel to every person would be cruel and hypocritical if Christ did not die for all sinners. Against those who make a travesty of Calvin's doctrine, he writes that "there is ready pardon for all sinners, provided they turn back to seek it" (Inst. III, 24, 16). The problem is that "no one seeks God", therefore Christ's sacrifice for all only becomes effectual in those God has elected from all eternity. {snip} Since "no one seeks God" Calvin asserts that "only those whom He has illumed do this"(Inst. III, 24, 17), and that God "does not indiscriminately adopt all into the hope of salvation but gives to some what he denies to others"(Inst. III, 21, 1).