Okay, finally found Iris' 'Happy' Ending and here's a tiny guide to get it:
Prerequisites:
1) Completing Titan's Step Quest (After checking for info in the Knight Hospitalier's camp, the player has to go back to Titan's Step and report to the Guild there.);
2) Completing the first part of Iris Quest;
Steps:
1) Once prerequisites are completed, you will find a new NPC (dark-haired kunoichi twin) standing by the first room by the entrance of Iris' hotel. Speak with her to start the Part 2 of Iris Quest;
2) Go to the castle-like building with four guards standing on the entrance, enter through and let the dialogue go through;
3) Go to the Church of Mara, the large castle-like building with no guards and let the Happy Ending unfold;
4) At the end of the scene, the seer will appear and say that this finale is 'incorrect' and will give you a choice - A) Agree (Reset to before Iris enters the Church of Mara and brings her back to the hotel) or B) Disagree (the game ends and you are brought to the main menu);
From what I got, Step 1 and 2 are official, while Step 3 and more are for the 'Happy' Ending alone. Once the new additions come to this storyline, there will be more options after Step 2 which will delve deeper into the storyline.