In Northern Ireland the ragwort law is written in the Noxious Weeds (Northern Ireland) Order 1977. This piece of legislation is simply a copy of the Legislation in force in the rest of the UK, namely the Weeds Act 1959 with minor changes to reflect practice in Northern Ireland. Just like the Weeds Act, this law does not make it compulsory to control ragwort. Orders may be made, but in the absence of one of these rare orders there is no requirement on anyone to do anything to control ragwort.
This is the important and relevant part of that Order:-
(1) Where an authorised officer is satisfied that there are noxious weeds of any kind growing upon any land, he may serve on the occupier of the land, or on any person who by virtue of an agreement has a right to the exclusive use of the land for cropping or grazing, a notice in writing requiring him—
(a)to cut down and dispose of, or otherwise destroy, those weeds in the manner specified in the notice; or
(b)where it appears to the officer to be expedient to do or refrain from doing any other thing for the purpose of preventing the spread of weeds of that kind on or from that land, to do or refrain from doing that thing.
It also creates a schedule of listed plants
Wild oat: Avena fatua L. Avena ludoviciana Durieu Thistle: Cirsium vulgare (Savi) Ten. Cirsium arvense (L.) Scop. Dock: Rumex obtusifolius L. Rumex crispus L. Ragwort: Senecio jacobaea L.
As documented elsewhere on this website, people frequently get the law wrong. The legislation is frequently misconstrued as meaning there is a legal obligation to control ragwort where it is absolutely clear that no such obligation exists.
Civil servants have been known to do just that and make a mistake about the law. On 7 June 2016 the Department published a web page "Destroy noxious weeds now" ,which erroneously stated that "Under the Noxious Weeds (Northern Ireland) Order 1977 ragwort, thistle, dock and wild oat are defined as noxious weeds and landowners have a legal responsibility to prevent the spread of these weeds."
A complaint was made by a member of the public and the department confirmed that the complaint was correct by saying, "The Noxious Weeds (Northern Ireland) Order 1977 does define ragwort, thistle, dock and wild oat as noxious weeds, but you are correct that the legal obligation on the occupier of land is in respect of compliance with a statutory notice issued by an authorised officer of the Department, not simply the presence of the weeds themselves. We will amend our publications to make this clear.