Red Iris Flowers grow in Pre-Searing Ascalon and in the Charr Homelands. They can be used to befriend the young Gwen, to exchange for various pre-Searing items, to obtain a Tapestry Shred, and in various EoTN quests related to Gwen.
A fixed number of Red Iris Flowers spawn when you enter an Explorable Area; they can be picked up by any party member. They are plentiful in Pre-Searing Ascalon and somewhat harder to find elsewhere. Each Iris appears randomly within a smaller dropzone of the area, so the flowers can be easy to find once you identify those dropzones.
You can effectively farm Red Iris Flowers in Pre-Searing, by exiting Ashford Abbey and look for the one that appears in Ashford Village (there is always exactly 1), and in Post-Searing by running to Gwen's Garden where there are always exactly 2.
The Northlands (3): near the Gate; near the east-most bridge (close to Ascalon City); near the most northern Charr mini-altar (due West of the empty Piken Square and just west of the northern bridge)
Outside Pre-Searing Ascalon: Irises can be found in Charr terriory and near the EoTN outpost:
The quickest place to farm irises in pre-searing is in Lakeside County; you can find the 5 flowers there in less than 5 minutes, if you ignore the mobs. Outside of pre-searing, you can grab the two irises in Gwen's Garden in less than 2 minutes.
To make it easier to find irises, add a keyboard shortcut to Target Next Item and spam that key when you get close to the iris-spawning part of a zone. Alternatively, using the default keys, keep the [Alt] key pressed as you explore and frequently change the direction you are facing.
To add a keyboard shortcut, open the Options window (default key: [F11]), switch to the Controls tab, find the command, Target Next Item, press a key (e.g. [PageDown]), and click on the assign button.
The pre-Searing cost of the the Small Equipment Pack is 50 flowers; the outside-Pre merchant price is 2 500 . Consequently, some players consider the flowers to be worth no more than 50