Tending to grow low across a dunes, the red stems are a diagnostic feature.
It's a species in the genus Euphorbia, which contains 1603 species and belongs to the Euphorbiaceae family. The plants bloom from June to August (this photo was taken in late August at Cullenstown, Co. Wexford).
The flowers are aranged as cymes - a more or less flat-topped flower-cluster. Don't confuse it with sea spurge (Euphorbia paralias) which is taller, with larger, less yellow flowers. The bracts of Portland Spurge (those modified leaves associated with the flower) are "ace of spades" shape with a tiny point at the end.