This is an annual or perennial plant which grows on wet mud, in brackish water, ponds and damp ditches. It has no floating leaves, but has submerged, finely divided and – as its name would suggest – thread-like leaves which fan out and branch repeatedly. The flowers (6–10 mm) are white with yellow centres, have five well-spaced petals and crescent-shaped nectaries. They flower in May and June, are not too commonly found around the country. They are native plants and belong to the Ranunculaceae family.
I first found this plant growing on Dublin's North Bull Island in 2010 when I also photographed it.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre