On the Mac side, you will use the Network Control panel, on your Apple TV, you will use the Network settings.
Make sure you know what your current settings are, on both your Mac AND the Apple TV.
Use an ethernet cable to connect your Mac to your Apple TV. Ideally you should use a crossover cable, but a straight through cable MIGHT work also.
Open the network control panel on your mac. Click on the Built in Ethernet on the Left. Change the "Configure" drop down to "Manually", and enter an IP address of 192.168.1.1. Click Apply. This will give your ethernet interface a local IP address. Use the Airport icon in the menu bar to turn off your airport. Now you should only have network connectivity on your ethernet connection.
On your Apple TV, go under "Settings/network", and select "Configure TCP/IP". Select Manually, and enter an IP address of 192.168.1.2.
Open Terminal (in Applications) on your Mac and at the command prompt, type this command:
ping 192.168.1.2
You should see lines repeating that say something like "64 bytes from 192.168.1.2...". Press CTRL-C to stop the display. This means you are now networked over the ethernet cable. You can close the terminal.
If you don't get any repeating response, then you have a network configuration or connectivity issue. Check your cables and IP addresses.
Once you can ping the Apple TV from your mac, launch itunes and see if you can sync your library. It should go much faster, since it is now going over the ethernet.
When you get your library copied, then unplug the cable, and put your Apple TV network settings back, and put your Network Control panel settings back. Finally turn on your airport from the menu, and see if you can get back on the internet from your mac.
Launch itunes and see if you can see your Apple TV.
I know this probably sounds complicated, if you haven't done much (any?) networking before.
Just make sure you have your original settings, so you don't mess up your existing connections when you are done...