There are 302 species of cnidarians (phylum Cnidaria) recorded in Ireland.^[1]

The cnidarians' distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic).^[2][3][4]

Cnidarians found in Ireland and Irish waters include sea pens, sea anemones, hydroids, sea jellies ("jellyfish") and corals.

• Actinia equina (beadlet anemone)
• Actinia fragacea (strawberry anemone)
• Anemonia viridis (snakelocks anemone)
• Anthopleura ballii (red speckled anemone)
• Aulactinia verrucosa (gem anemone)
• Urticina eques
• Urticina felina (northern red anemones, dahlia anemone)

• Stomphia coccinea

• Aiptasia mutabilis

• Capnea sanguinea

• Edwardsia claparedii
• Edwardsia delapiae
• Edwardsia timida (timid burrowing anemone)
• Edwardsiella carnea
• Scolanthus callimorphus

• Gonactinia prolifera

• Halcampa chrysanthellum

• Halcampoides elongatus

• Mesacmaea mitchellii
• Peachia cylindrica

• Adamsia carciniopados
• Adamsia palliata (cloak anemone)
• Calliactis parasitica
• Cataphellia brodricii
• Hormathia coronata

• Metridium senile (plumose anemone, frilled anemone)

• Actinothoe sphyrodeta
• Cereus pedunculatus (daisy anemone)
• Phellia gausapata
• Sagartia elegans
• Sagartia ornata
• Sagartia troglodytes (mud sagartia, cave-dwelling anemone)
• Sagartiogeton laceratus
• Sagartiogeton undatus

• Alcyonium digitatum (dead man's fingers)
• Alcyonium glomeratum (red sea fingers)
• Alcyonium hibernicum (pink sea fingers)
• Parerythropodium coralloides

• Eunicella verrucosa (broad sea fan, pink sea fan, warty gorgonian)

• Swiftia pallida (northern sea fan)

• Arachnanthus sarsi

• Cerianthus lloydii (lesser cylinder anemone)
• Pachycerianthus multiplicatus (firework anemone)

• Corynactis viridis

• Virgularia mirabilis

• Epizoanthus couchii

• Isozoanthus sulcatus
• Parazoanthus anguicomus
• Parazoanthus axinellae (yellow cluster anemone)

• Bougainvillia muscus
• Garveia nutans

• Candelabrum phrygium

• Corymorpha nutans

• Coryne eximia
• Coryne muscoides
• Sarsia densa

• Eudendrium annulatum
• Eudendrium arbuscula
• Eudendrium ramosum

• Clava multicornis
• Hydractinia echinata (snail fur)

• Hydra viridissima (green hydra)^[5]
• Hydra vulgaris (common hydra)^[5]

• Leuckartiara octona
• Neoturris pileata

• Velella velella (sea raft, by-the-wind sailor, purple sail, little sail)

• Ectopleura larynx
• Tubularia indivisa (oaten pipes hydroid)

• Aequorea forskalea
• Aequorea vitrina

• Aglaophenia acacia
• Aglaophenia kirchenpaueri
• Aglaophenia pluma (toothed feather hydroid, podded hydroid)
• Aglaophenia tubiformis
• Aglaophenia tubulifera
• Gymnangium montagui
• Lytocarpia myriophyllum

• Campanularia hincksii
• Clytia hemisphaerica
• Laomedea angulata
• Laomedea flexuosa
• Obelia dichotoma
• Obelia geniculata
• Obelia longissima
• Obelia plicata
• Rhizocaulus verticillatus

• Halecium beanii
• Halecium halecinum
• Halecium muricatum
• Halecium plumosum

• Antennella secundaria
• Halopteris catharina
• Schizotricha frutescens

• Kirchenpaueria pinnata

• Lafoea dumosa

• Nemertesia antennina
• Nemertesia ramosa
• Plumularia argentea
• Plumularia cupressina
• Plumularia distans
• Plumularia setacea (plumed hydroid, little sea bristle)
• Polyplumaria flabellata

• Abietinaria abietina
• Abietinaria filicula
• Amphisbetia operculata
• Diphasia alata
• Diphasia attenuata
• Diphasia fallax
• Diphasia nigra
• Diphasia pinaster
• Diphasia rosacea
• Dynamena pumila
• Hydrallmania falcata
• Sertularella fusiformis
• Sertularella gayi
• Sertularella mediterranea
• Sertularella polyzonias
• Sertularella rugosa
• Tamarisca tamarisca
• Thuiaria articulata (jointed hydroid)
• Thuiaria thuja

• Craspedacusta sowerbii^[6]

• Apolemia uvaria (string jellyfish, barbed wire jellyfish, long stringy stingy thingy)

• Physalia physalis (Portuguese man-o'-war, blue bottle, floating terror)

• Rhizostoma pulmo (barrel jelly, dustbin-lid jelly, frilly-mouthed jelly)

• Cyanea capillata (lion's mane jelly)
• Cyanea lamarckii (blue jelly, bluefire)

• Chrysaora hysoscella (compass jelly)
• Pelagia noctiluca (mauve stinger)

• Aurelia aurita (moon jelly, common jelly, saucer jelly)

• Caryophyllia inornata
• Caryophyllia smithii (Devonshire cup coral)
• Lophelia pertusa^[7]

• Madrepora oculata (zigzag coral)^[7]

• Craterolophus convolvulus

• Lucernariopsis campanulata

• Haliclystus auricula (kaleidoscope jelly)

• World Register of Marine Species

• Marine species identification portal