In ~/.ssh/config, add:
***
Host github.com
HostName github.com
IdentityFile ~/.ssh/id_rsa_github
***
If the config file is new, you might need to do chmod 600 ~/.ssh/config
Now you can do git clone
git@github.com:{ORG_NAME}/{REPO_NAME}.git
Where {ORG_NAME} is your GitHub user account (or organization account)'s GitHub URI name.
Note that there is a colon : after github.com instead of the slash / - as this is not a URI.
And {REPO_NAME} is your GitHub repo's URI name
For example, for the Linux kernel this would be git clone
git@github.com:torvalds/linux.git).
NOTE: On Linux and macOS, verify that the permissions on your IdentityFile are 400. SSH will reject, in a not clearly explicit manner, SSH keys that are too readable. It will just look like a credential rejection. The solution, in this case, is:
***
chmod 400 ~/.ssh/id_rsa_github
***