Linux repositories

You have to use HTTP (not FTP) and go through the Proxy . I don't know much about Ubuntu's Update Manager, and recommend you try Synaptic: in there you go to Settings / Preferences / Network / Manual Proxy - and enter proxy.ucd.ie port 8484.

optional for the geeky: If you want to use terminal commands such as apt-get, the easiest way is to create a settings file. I put the following in ~/ucd.conf:

Acquire::http::proxy "http://proxy.ucd.ie:8484/";
Acquire::ftp::proxy "ftp://proxy.ucd.ie:8484/";
Acquire::https::proxy "https://proxy.ucd.ie:8484/";

Then I can run:
sudo apt-get -c ~/ucd.conf update
to update from the repositories etc.