What is the difference between Irresistible Grace and the Immaculate Conception? So, there's a Calvinist dogma called "Irresistible grace" which I take to mean that some people are unable to resist the call of God and will do the "Godly thing" against their freewill (or instead of their freewill).  

And then there is The Immaculate Conception, which is the Doctrine that The Blessed Virgin Mary was conceived without Original Sin, thus giving her the power to follow her freewill perfectly and not the normal corrupt passions that drag the rest of us down.

So, according to the Catholic Church, why do we say she had freewill instead of saying she submitted to irresistible grace? 

Also, is there a sense that some of the Apostles and/or Patriarchs got to a point where they acted as if they were following their conscience perfectly, the way Our Lady did?
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).