Add this script to your ~/.bashrc (~/.zshrc, ~/.bash_profile etc) and it'll ask you if you want to run irssi when you start a terminal, unless irssi is already running. The script automatically chooses the default after a timeout (currently 3 seconds). Pressing return or n/N will skip it.

.zshrc

# Run irssi if it's not already running
ps cax | grep irssi > /dev/null
if ! [ $? -eq 0 ]; then
  yn="y" # Set the default, if the timeout expires
  echo -n "Run irssi? [yn] "
  read -t 3 yn
  case $yn in
    [Yy]* ) irssi;;
    * ) ;;
  esac
fi