Coenagrion lunulatum (Charpentier, 1840), the Irish Damselfly or Crescent Bluet, is a damselfly found in northern Europe; outside northern Finland the species is rare. It is also scarce and local in the Netherlands and Ireland. One of its English name comes from the fact that it is found in Ireland but not in Britain. The alternative name, Crescent Bluet, refers to the shape of the markings on segment two of the male and its scientific name. The black mark on the male's S2 is in the shape of a half-moon crescent, although this mark is not always very recognizable in the field. More readily observable are the species' greenish tinge, the green underside of the eyes and of the abdomen, the absence of a connection between the postocular spots in the male, and the colour of the abdomen, which - dorsally - is largely black and which, as a result, gives the insect a rather stocky look compared to most of its congeners. 

The species has a length around 31 mm. In males the back of the abdomen is mostly black, S2 usually has a small crescent shaped dark spot flanked by two black lines. S8 and S9 are blue except for 2 small black spots on each. The blue is of a darker shade than similar species. The black markings are fairly extensive giving the damselfly a dark appearance. The underside of the male is bright green especially on the head and thorax. The female is apple green and black but with blue markings on S8-10. The hind margin of the pronotum has a prominent raised point.

This species is similar to the Azure Damselfly (C. puella), the Variable Damselfly (C. pulchellum) and the Common Blue Damselfly (Enallagma cyathigerum) with which it coexists. However, both the female and male are darker and shorter-bodied in comparison with other blue damselflies. Also, look at S2 (on the males) for distinguishing characters.

Coenagrion lunulatum is a predominantly North-Eastern species with only a few outposts in the West, notably in Ireland, France and the Netherlands. The Irish population is thought to be one of the largest in western Europe. It also occurs through Siberia to Mongolia and Japan. 
 
 

.