The fortress of the Ire was presumably in the space between Scadrial and Threnody. Apparently "the powers of Threnody" wanted to "join the stage".
We see that Kelsier, a Cognitive Shadow was able to make his way that far, presumably the Threnodite Shades could too, unless the Ire were referring to others. We don't know much about Threnody, and that world has had some big changes.