This is a very straggly, weak little perennial which, as its name suggests, grows commonly in ditches and marshes. The pretty white 4-petalled flowers (3-4mm across) are carried in loose, open clusters on rough, angled stems and bloom from June to August. The leaves are narrow, with backward-facing bristles on their margins and are in whorls of 4-6. This is a native plant which belongs to the family Rubiaceae.
My first record of this plant is at Tacumshane, Co Wexford in 2006 and I photographed it there in 2007.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre