When can I get Eirena's conversations for the Secret of Ages achievement?

Eirena's Journey (Act II-IV) - Available after completing Shadows in the Desert (Part 12). Go back to town and if the enchantress was already your companion, hire another. Then this conversation should be available.
A Lost Age (Act II-IV) - Available after completing Shadows in the Desert (Part 12).
The Unanticipated (Act II-IV) - Available after accepting City of Blood (Part 1).
Missing Pieces (Act II-IV) - Available after accepting Unexpected Allies (Part 3).
The Prophecy Broken (Act II-IV) - Available after completing Betrayer of the Horadrim (Part 7).
Ancient Secrets (Act II-IV) - Available after accepting The Black Soulstone (Part 1).
The First Memories (Act II-IV) - Available after accepting Lord of Lies (Part 5).
The Vizjerei Lord (Act III-IV) - Available after accepting The Siege of Bastion's Keep (Part 1).
A Sacred Pact (Act III-IV) - Available after accepting Machines of War (Part 1).
Leah's Fate (Act IV) - Available after accepting Fall of the High Heavens (Part 1).
A Lost Friend (Act IV) - Available after accepting Fall of the High Heavens (Part 1).
The Dying Angel (Act IV) - Available after accepting The Light of Hope (Part 1).
A Hidden Legacy (Act IV) - Available after accepting Beneath the Spire (Part 1).

As all the followers appear before you after you defeat the final boss, almost all of their conversations are available until the end of the game, but there's one that can be missed so I'll mark it with a +, but there's a strange special case one that I'm noting with a ?. Many of the conversations will unlock only after having heard other previous conversations, but will not become available until the point specified. Also, some of the follower conversations may be hidden if they are not following you, but by having them follow you, you can get a complete list (without the extras).

Act I

A Lost Age: Available after accepting the quest The Fallen Star until the end of the game.

Act II

? Eirena's Journey: Available from the time after you begin Act II and before you accept the quest Shadows in the Desert until you begin Act III. This one only appears if Eirena has been out of the party (not following) at least once so if you choose to have her follow you when first asked to do so, this conversation won't show up until she stops following it seems.
The Unanticipated: Available after accepting the quest City of Blood until the end of the game.
Missing Pieces: Available after accepting the quest Unexpected Allies until the end of the game.
The Prophecy Broken: Available from the time after you complete the quest Betrayer of the Horadrim and before you accept the quest Blood and Sand until the end of the game.
Ancient Secrets: Available after accepting the quest The Black Soulstone until the end of the game.
The First Memories: Available from when you are given the objective Talk to Tyreal at the Hidden Camp during the quest Lord of Lies until the end of the game.

Act III

The Vizjerei Lord: Available from the time after you begin Act III and before you accept the quest The Siege of Bastion's Keep until the end of the game.
A Sacred Pact: Available after accepting the quest Machines of War until the end of the game.

Act IV

Leah's Fate: Available after accepting the quest Fall of the High Heavens until the end of the game.
+ A Lost Friend: Available during the quest Fall of the High Heavens.
The Dying Angel: Available after accepting the quest The Light of Hope until the end of the game.
A Hidden Legacy: Available after accepting the quest Beneath the Spire until the end of the game.

Extras

A Hero's Prophecy: Available from when you are given the objective Talk to Auriel and bring your quest to an end during the quest Prime Evil until the end of the game.
Fate's Fulfillment: Available from when you are given the objective Talk to Auriel and bring your quest to an end during the quest Prime Evil until the end of the game.