This is a really small species – 20cm tall at most – and it is also rare. It is a slender, erect, glabrous (hairless) annual which is found in dry stony ground, on walls and sandy soil. It bears 5mm flowers with white-edged sepals longer than the 5 white petals. The leaves are very narrow and pointed, about 8mm long. It is an introduced member of the Campion family (Caryophyllaceae) which flowers from June to August.
I first saw this species in June 2015 on a BSBI Field Trip in Co Kilkenny with Roger Goodwillie, BSBI Vice-county Recorder. It was in a disused quarry where the ground was very sandy.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre