Could you explain your situation in more detail please?
Including the extent of the damage and who the culprit is please?
As much detail as you have is really useful in helping experts advise you fully.
According to the roundup website, it will kill grass but “Roundup is inactive on contact with the soil. It is fixed to the soil particles and gets degraded by micro organisms in the soil into natural materials” so the soil is no issue.
If the neighbour has killed the hedge, that is criminal damage and is a police matter. If he inadvertently killed the grass or plants, that is not criminal damage because there is no criminal intent but it is nonetheless negligence and you are entitled to be recompensed and put back into the position that you would have been had his negligence not occurred.
Under the Nuisance Tree legislation you can be made to cut any nuisance hedge down to 2 m. The neighbour would bring the complaint and the local authority enforce it. The neighbour has to pay a fee of £300 to the local authority. If the local authority decide that the hedge is too tall and is a nuisance, they will order you to cut the hedge down to 2 m.
There is no agency that would deal with this. It is simply a case of getting a Gardner or horticultural expert to give you a quotation for putting everything right.
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In my experience, the Garda take this kind of thing far more seriously than the British police.
It would not be at all unusual for anyone charged with criminal damage (any criminal at all for that matter) to deny this.
A criminal prosecution has to be brought beyond all reasonable doubt but the burden of proof in a civil court just for the money (with no criminal prosecution) is on the balance of probabilities and in view of the fact that the neighbour has a history with regard to this and obviously you haven’t done it, then on the balance of probabilities, it’s the neighbour. Not enough for a criminal prosecution but certainly enough for civil litigation.