RPC calls to a specific client?

Now I understand for the most part how RPC’s work. The only issue i’m having is singling out a specific client. I think this code does that:

networkView.RPC("someFunction", Network.connections);
@RPC
function someFunction() { }

But I’m not sure how exactly to get the id for a specific client. How do I find the right connection?

But I'm not sure how exactly to get the id for a specific client. How do I find the right connection?

What do you mean by "right connection"? If you want to send the RPC to a client with a specific IP (C#):

foreach (NetworkPlayer player in Network.connections)
{
    if (player.ipAddress == matchingIp)
    {
        networkView.RPC("someFunction", player);
    }
}

You can probably figure out how to replace the comparison test with something more specific for your problem.

For your updated comment I think you need to do something similar like this in your code:

[RPC]
void OnLogin ( string username, string password, NetworkMessageInfo info )
{
    if ( Authenticate( username, password ) )
    {
        networkView.RPC( "OnLoginSuccess", info.sender );
    }
    else
    {
        networkView.RPC( "OnLoginFailure", info.sender );
    }
}