Who is ignorant and when does one realize that one is ignorant? Who is considered to be ignorant according to Buddhism? What does the term ignorance cover ? When does one realize, one is ignorant ?

Can one know nothing of ignorance and still become enlightened?
everyone who is subject to samsara or the round of rebirths is ignorant, because


  From ignorance as a requisite condition come fabrications.
  From fabrications as a requisite condition comes consciousness.
  From consciousness as a requisite condition comes name-&amp;-form. 
  From name-&amp;-form as a requisite condition come the six sense media.
  From the six sense media as a requisite condition comes contact.
  Fromc ontact as a requisite condition comes feeling.
  From feeling as a requisite condition comes craving.
  From craving as a requisite condition comes clinging.
  From clinging as a requisite condition comes becoming.
  From becoming as a requisite condition comes birth.
  From birth as a requisite condition, then aging &amp;  death, sorrow, lamentation, pain, distress, &amp; despair come into play.


Paticcasamuppadavibhanga sutta ( SN 12.2)

i think ignorance covers nescience of the phenomenon of suffering and the way of escape from it


  When does one realize, one is ignorant ?


perhaps once one realizes that they suffer while having no clue how to avoid that


  Can one know nothing of ignorance and still become enlightened?


i find this an interesting  question, my guess is 'NO', but i'm not sure i can explain why i think so and can't remember suttas which explain this point.
that's as far as awakening is concerned

as for nibbana i'm a bit more certain, just as suffering is effected by a  chain of causes, quoted above, so is nibbana, for example


  What is impermanent is suffering. What is suffering is nonself. What
  is nonself should be seen as it really is with correct wisdom thus:
  ‘This is not mine, this I am not, this is not my self.’
  When one sees this thus as it really is with correct wisdom, the mind becomes dispassionate and is liberated from the taints by nonclinging.
  By being liberated, it is steady;
  by being steady, it is content;
  by being content, he is not agitated.
  Being unagitated, he personally attains Nibbāna.


Anicca sutta (SN 22.45)

an aspirant needs to go through a series of stages of mind development to arrive at that destination and those cannot be bypassed

the first step in curing an illness is its acknowledgement