This bushy perennial or biennial could be mistaken for Weld at a quick glance and whilst it is related to that plant there are several clear differences. Wild Mignonette is also an upright plant but it only reaches about 50cm high and its stems are solid, not hollow as with Weld. The flowers (6-8mm across)are yellow-green and borne in a compact spike from June to August. They each have 6 petals, the upper divided, up to 20 stamens and a somewhat raggedy appearance. They are unscented but much loved by bees and are the larval foodplant for the Large White butterfly. The leaves are pinnately lobed and have wavy edges. The fruit is an oblong pod. This wildflower is not as widespread as Weld, is an introduced plant and it belongs to the family Resedaceae.
Along with Weld, I first recorded this plant at Dalkey Railway Station, Co Dublin in 2007 and photographed it there the same year.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre