Android smartphones needs to be rooted to perform many tasks and unlock several features or options that were originally locked and restricted by the phone. Even Droid Eris got rooted finally by Dev team and jcase at XDA forums. If you want to get your Droid Eris rooted, you need to follow a simple tutorial and use the Droid Eris root image file. Here is the step by step tutorial on how to do this.
First things first. Download Droid Eris 1.5-2.1 root image for free from androidmobileos. Take a backup of your phone before you start rooting Droid Eris smartphone.
How to Root Droid Eris Android 1.5 – 2.1
- Keep the PB00IMG.zip in the root of your sdcard
- Backup all important data with Google
- Turn off your phone
- Turn it on again by pressing ‘power+volume down’ keys simultaneously
- Wait till your phone loads PB00IMG.zip
- It will ask to press action to update, then press trackball center
- It might take up to 15 minutes to boot the first time
- Now go through the Google setup wizard
- You are done.
If you face any problem after rooting Droid Eris phone such as HTC widgets missing or sense not working, etc, reload the img and repeat steps 3-8 to try the update process again.
To Mount use: mount -o rw,remount /dev/block/mtdblock3 /system
P.S: By rooting your phone you may void your warranty or brick your phone. Do it at your own risk, we will not be responsible if your phone renders unusable after following above guide.
Below are some more related articles to the topic:

Will this reset my phone and wallpaper?
Erase my apps?
Should I use a blank SD card for the zip file? Or my current one with music and ROMs on it?