Companion to English ivy, under a large tree...
Are there any plants that will survive or even thrive under this tree (which the 'Seek' app says is A Northern White Cedar) alongside the English Ivy that is already there?
Ps, the tree isn't growing out the side of a cliff... Not sure what happened to the photo!
A large tree and Ivy will inevitably use the available ground showing in your photo. Both will suck up any available rainfall and nutrients very quickly.
If you want to plant anything else, consider extending/improving the ground and perhaps try some of the shade loving perennials and bulbs