Her fate rests in your hands.
Irina of Carim is a new NPC you can interact with in Dark Souls III. To free her from her cell in the Undead Settlement, you need to give the Shrine Maid the Mortician’s Ashes and buy the Grave Key. This unlocks the gate in the sewers, and gives you access to where Irina is being held. Exhaust all dialogue with her and she’ll head back to Firelink Shrine. Exit the area and exhaust your dialogue with Eygon as well.
To get the ‘good’ ending to her questline, you have to give Irina divine tomes and keep the dark and deep tomes away from her. This means that you can only give her the Carim and Lothric braille tomes, and not the Deep and Londor tomes. After you’ve given her those tomes, buy all the miracles she has for sale, and she’ll be able to become a proper Firekeeper. She’ll then relocate outside the tower behind Firelink Shrine.
However, if you give her the dark tomes and purchase dark miracles from her, she’ll soon disappear from Firelink Shrine. You can find her and Eygon outside the Iudex Gundyr boss arena, and you’ll have to fight Eygon to the death. After killing Eygon, speak to Irina behind him. Speak to her once, and then wear Eygon’s gauntlets. She’ll mistake you for Eygon and ask you to kill her as well.
Be sure to check out our Dark Souls III wiki for more tips and tricks.