I am tethering a laptop to a cell phone for internet connection, and I can connect to most IRC networks I have tried, but can't connect to Undernet. I am trying straight to IP rather than by name and still have the problem: a typical Connection Refused error.

Anyone have similar problems or is this a known issue specific to Undernet & tethering?

Certain cell phone networks, such as Verizon, block a lot of IRC network connections. Usually the IP works, but not always. That may be your problem.

The problem is limited to Undernet specifically. I have jumped on DALNet and one other network.

They probably block IP's used by ISP's for cellphones to prevent spam abuse since the IP's are extremely dynamic and you can easily get a new IP when you reconnect to the internet.

A few networks do this.