"tire thy brains to gain a deity...."
See in text (Scene 1)
This statement is ironic. While Faustus believes that he will become a "deity" by practicing necromancy, in reality the only "deity" he will gain is the Devil.
See in text (Scene 3)
Faustus's grand ideas of enacting whatever magic he wishes and commanding ultimate power in the world here devolves into dedicating himself to a higher power. Ironically, thought Faustus initially turns to black magic because he believes it will give him the most power, it takes away all of his power and makes him a servant to the devil.
"This will I keep as chary as my life..."
See in text (Scene 5)
"Chary" means dearly or carefully. This statement is ironic because Faustus did not keep his life or his soul carefully.
"transform him straight...."
See in text (Scene 9)
This is another instance in which we see Faustus unable to execute his own magic. He controls Mephistophilis, but wields no power of his own. Ironically, Faustus sells his soul for power and gains none.