If the public IP address has an SSH server and you have login credentials for it, you can set up SSH port forwarding.
In command line, you can do this:
ssh firstname.lastname@example.org -L 22:192.168.x.x:22
This sets up a port forwarding from your own computer to the server in the internal network. user is the user account at the server having 61.xx.xx.xx address, and first 22 is the port on your local computer that you connect to when you want to connect to the server inside the private network.
connects you then to the other server. If you already have SSH server running on your local computer, you can use another port like 222, then you connect like this:
ssh -p 222 email@example.com
You need to adjust the port in the first SSH command that sets up the tunnel.
If you need to set up tunneling with PuTTY, http://howto.ccs.neu.edu/howto/windows/ssh-port-tunneling-with-putty/ has clear instructions for that.
If there is no SSH daemon running on the 61.xx.xx.xx address, then you need to configure port forwarding on the router that handles traffic between public and private network. You need to configure port forwarding from some external port (61.xx.xx.xx:222 for example) to the internal IP 192.168.x.x:22.