This feature is available since AndroIRC 3.0
Starting with AndroIRC 3.0, username can be auto-completed for you easily. All you need to do is to type the first letter of the username, and use the Search button of your phone to auto-complete the username. If there're multiple choices of auto-completion, just user the Search button again to cycle through all the possible choices. If you have a physical keyboard on your device, you can also use the Tab key to auto-complete.
Starting with AndroIRC 3.1, you can also auto-complete channels name. Just enter
# and use the auto-completion key, and AndroIRC will auto-complete using the list of all opened channels. The same rules as for usernames apply: use the auto-complete key to cycle between all available possibilities.
Honeycomb and Ice Cream Sandwich devices don't appear to have a physical Search button. If AndroIRC is used on these version of Android, a magnifying glass icon will appear on the left of the text box. Click on this icon to use the auto-completion.