Wait-- vast majority? Not according to that Wikipedia article on the Irish language you posted, not even close.
The wiki article is dependent on answers people give on their census forms, and I know from personal experience people tend to underestimate their level of Irish and claim none when they know some, and some when they're fluent.
And Wales?
Dunno, sorry, you'll have to ask a Welsh person.
In any case I still dont think thats cause for it to be compulsary, think there are other factors at hand.
Of course there are other factors at hand, mainly that the Irish don't want their language to join the many minority languages due to become extinct in the next century. I was just reacting to your trivialisation of what you quaintly describe as "gaelic". Irish is a minority language, not a dead language, it is still used in daily life over here.