The Large Red Damselfly is one of the most common damselflies and often the first damselfly to emerge, usually in April or May.
Mature males have red shoulder stripes.
Females and immature males have yellow shoulder stripes.
The edges of the segments in both females and males are black but all females also have yellow/orange bands around the abdominal segments which are absent in males.
Males have black only on the last 4 segments of their abdomen while all of the abdominal segments of females are partly (and variable) black.
Mature females occur in three colour forms: typical, fulvipes and melanotum.
The form fulvipes's upperside of its last 3 abdominal segments are entirely black.
The form typica's upperside of its last 4 abdominal segments are entirely black.
The form melanotum's upperside of all of its abdominal segments are predominantly black.

They can easily be confused with the Small Red Damselfly (Ceriagrion tenellum), but the latter has orange legs, while the Large Red Damselfly has black legs.

The first 3 photos show a male with a somewhat broken abdomen.
The last photo shows a very rare mating ritual between a red male Large Red Damselfly and a female Red-eyed Damselfly (Erythromma najas).

Annamoe Trout Fishery, Wicklow, Ireland, 9 June 2018