This really depends on where you live. A few coco's have it as their policy that no burning of any waste should take place, this makes it easier to enforce the regs. The policy is based upon the individual coco's interpretation of the air pollution act as well as the waste management act. Most of the people involved in enforcement have some sort of environmental nous, and know that the burning of green waste like brush, branches, grass whatever is not going to cause environmental pollution, but it may cause nuisance, which is also something to consider, depending on where one may intend to burn of the waste. But if the coco in question does not allow the burning of any waste, it makes it easier for the chaps on the ground to enforce, as in, if theres any fire, its a case of 'put it out or else'. If they were to allow burning, then 9 times out of ten, the burner would be sneaking in a few bags of this and the old buckets from up the back etc. If you're caught in the act of burning bushes or whatever the green material might be, even if you are in an area who policy is not to allow it, you would be terribly unlucky to be prosecuted for it, as it would be pretty difficult to prove that there was serious environmental pollution caused as a result of burning only green material, and anyway, as I said most of those involved use their discretion.
Also, in order to clear this situation up, I think there are new regs. on the way which will include provision for the burning of green waste under permit (coco will come out and have a look before you burn, and issue permit), while also introducing an On The Spot fine of €500 for those who are caught burning materials likle plastic etc., which is certainly going to cause env. pollution if burnt. This means they don't have to go to court, the fine is issued, and if they want to appeal, then they can go to court and risk larger penalties.