Whenever you want to use a custom identity file with ssh, you usually use the -i parameter.
$ ssh -i ~/.ssh/other_id_rsa firstname.lastname@example.org
This will validate against ~/.ssh/otheridrsa.pub and not the default one at ~/.ssh/id_rsa.pub.
If you want to use this with the default git client, it won't work. You cannot specify the identity file when cloning/pulling from or pushing to a repository, yet.
$ git remote add origin ssh://email@example.com:repository.git $ git pull origin master
will fail, because it's using the default rsa key at ~/.ssh/id_rsa.
Instead you can use this workaround.
Adjust your ~/.ssh/config and add:
Host example Hostname example.com User myuser IdentityFile ~/.ssh/other_id_rsa
Now use the ssh host alias as your repository:
$ git remote add origin example:repository.git $ pull origin master
And it should use the otheridrsa-key!