This is an erect, almost hairless annual which grows to about 50cm high, with branching stems. It is found mainly on waste ground, gardens and cultivated soils. It bears spikes of small 2-4mm, yellowish-green, almost-stalkless flowers, on separate-sex plants, from July to October. It has shiny green, narrow-ovate, toothed leaves. Its fruit is bristly. Annual Mercury is an introduced species which belongs to the Spurge or Euphorbiaceae family, and is now mainly found in the south and east of Ireland.
I first spotted this wildflower in Sallynoggin, Co Dublin in October 2014 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