Des didn't learn it for the good of his health. He made a TV show which presumably earned him lost of money.
But seeing as you take the knee-jerk reaction of asking why our public servants don't learn Irish, here's a few;
1) They are kind-of busy with the day job. What loss in productivity of public services would you be prepared to live with to give people the time to learn a new language?
2) Taking money off people would be a breach of existing terms and conditions and would be illegal. It's generally not a good idea for the Govt to take illegal actions.
3) The vast majority of the Irish population are very likely to continue to want to interact with the public service in English, therefore the value-for-money return in spending time and cost on all public servants learning Irish is highly questionable.
4) Forcing people to learn a language is generally not very effective. They've tried it that way with Irish in schools, and it hasn't worked. Why would you want to continue to go down this road?
Maybe if you sit back and think about this for more than a few seconds, you'll come up with some constructive suggestions.