NiallBeag wrote:
In Scottish Gaelic, I believe the natural opposite is along the lines of "bu choir nach dèan thu sin", making the negative refer to the action, not the obligation. (Consider that "you don't have to do that" indicates a lack of obligation, not an obligation against action.)
Is there a similar form in Irish?
There is, but it may be limited to bureaucratic contexts. At least, the majority of Ghits I'm finding for "Ba chóir nach..." are from official documents (most representing the consecrated phrase "Ba chóir nach rachadh na haighneachtaí sin thar X focal), with only a handful from personal blogs and the like. Ar chuala aon duine an leagan cainte seo i mbéal an phobail?