Run this script from Cron:

SERVER=irc.freenode.org
CHANNEL=mychannel
NICK=mynick

git pull
git log -4 --pretty=format:'%h - %an : %s (%cr)' > commits.txt

function announceCommits(){
  (
    echo NICK $NICK
    echo USER $NICK 8 \* : Notifier
    #echo "JOIN #$CHANNEL"

    while read -r LINE ; do
      #echo "PRIVMSG #$CHANNEL $LINE" >&2
      echo "PRIVMSG #$CHANNEL :$LINE"
      sleep 1
    done < commits.txt
    echo QUIT
  ) | nc -v $SERVER 6667
}

announceCommits

What's missing is detection of last announced commit.

That would go along the lines of keeping the last announed commit hash in a file, and only dumping the successors.

That should work with `git log --since`, but doesn't. I didn't want to spend more time fixing that as Jenkins does this for me (git poll build trigger).