This information is available elsewhere, but placing here for easy access.
I recently had the need to use a specific key file for an SSH connection. In this case it was for a git server that needed different credentials than I use for other hosts.
It turns out it is very easy to configure SSH to use a different key when connecting to a specific host. Just add the following to your ~/.ssh/config file.
Host git.host.name Hostname git.host.name IdentityFile ~/.ssh/special_key IdentitiedOnly yes
That’s it. Now when attempting to establish an SSH connection to git.host.name, SSH will automatically know to use the key file specified for that host.