When you have completed all you wish to do in Udûn you should speak to Gimli and Ingold about Dor Amarth.
'You have been a great help, in the wake of Lachír's folly. You have helped lead us to conquer what resistance and pitfalls we have encountered here in Mordor.
'Some of our company has pressed through, past the gates of Carach Angren, and into Dor Amarth. When you have accomplished all that you wish to do here with us in Udûn, those in Dor Amarth could use your aid.'
The Ruins of Díngarth is located near Udûn in Dor Amarth.
You should travel to the Ruins of Díngarth.
Gimli and Ingold can be found at the Ruins of Díngarth in Dor Amarth.
When you have completed all you wish to do in Udûn you should speak to Gimli and Ingold.