I am not sure about special things with Trial of Iron saves, but the official save locations are:
%USERPROFILE%\Saved Games\Pillars of Eternity
%USERPROFILE% usually is something like C:\Users\your-name-here
For Steam, GOG, and DRM-free backer disc versions:
~/Library/Application Support/Pillars of Eternity/Saved Games
If you bought Pillars of Eternity from the Apple App Store, your save game directory is located here:
~/Library/Containers/com.mpdigital.pillarsofeternity/Data/Library/Application Support/Pillars of Eternity/Saved Games
If $XDG_DATA_HOME is not defined, it's stored in ~/.local/share
Steam cloud saves
The steam cloud saves functionality syncs your local save files with the steam cloud. Modifying your save file outside of steam will likely cause this functionality to pop up a warning message about the cloud and local versions being out of sync the next time you launch the game. You can find more information about the steam cloud in this gaming.se question.
It can be enabled/disabled by right-clicking on the Pillars of Eternity game in your library and selecting the "Properties" option, then going to the bottom of the updates tab and checking/unchecking the "steam Cloud Synchronization" box as in the below screenshot.