As Minister for Transport, Tourism & Sport, I have responsibility for overall policy and funding in relation to the national roads programme. Under the Roads Acts 1993-2015, the planning, operation and maintenance of individual national roads is a matter for Transport Infrastructure Ireland (TII) in conjunction with the local authorities concerned.
In relation to the use of weed killer on national roads, I can refer you to the TII document: 'Guidelines on The Management of Noxious Weeds and Non-Native Invasive Plant Species on National Roads' for information.
For a more detailed reply, I have referred the question to TII for a direct reply to you. Please advise my private office if you do not receive a reply within 10 working days.