In Ireland, the word we use for “monster” is “peist” (which is actually translated as ‘worm’ or ‘reptile’). That’s because our ancestors explained the creation of river routes with stories of giant, worm-like creatures who lived in waterways, being chased through the landscape by mythological heroes like Fionn mac Cumhaill (and later Christian Saints). In present day Scotland (which had the same cultural belief systems), that’s why traces of this belief system still linger with the story of Nessie in Loch Ness.
It’s possible they may have looked like this (but probably not!)
Interestingly, for an island, Ireland has a surprisingly limited number of references to sea monsters in the existing mythological literature and folklore (although there’s quite a lot relating to inland rivers). Naturally, there are tales aplenty involving seals (i.e. selkie). These are most common to the north and along the coast of current-day Scotland but, for the most part, the tales portray the creatures as relatively benign (i.e. they’re certainly not described as fearful or terrifying), probably because of their small size.
Basking Shark in Cork Harbour: picture from Evoke.ie (http://evoke.ie/news/were-gonna-need-a-bigger-boat-awesome-video-of-a-massive-shark-in-cork-harbour)
Back in the day, the native Irish population would certainly have been accustomed to seeing physically impressive animals such as the basking shark and the whale when they were at the water’s edge or travelled offshore. That’s probably why we have a relatively substantial body of Irish mythology around the míol mór (the whale). Such mythology tales are predominantly found in the immram (a name sometimes applied to Irish sea voyage stories) such as the Voyage of Maol Dúin or the Voyage of Saint Brendan the Abbot (Navigatio sancti Brendani abbatis) and can actually be quite entertaining. In the latter, for example, the sailing saint and his companions land on an island and set a fire to warm themselves. The island abruptly moves in response to the great heat of the fire and turns out to have been a slumbering giant whale.
Irish stories of sea creatures would also have been influenced in later times by the mythology of other cultures introduced as a result of invaders (e.g. the Vikings, Normans etc.), traders and Christians. The Normans for example, are believed to have brought numerous stories from Greek mythology over with them which merged over time with the local tales (e.g. Irish stories of mermaids are predominantly based on this). Again, however, even the merged tales do not usually incorporate sea monsters.
On a more local basis, the stories of sea creatures would almost certainly have been supported by the discovery of carcasses along the Irish coastline and the incorporation of such facts in to the tales of the native population. Whales certainly washed up on a relatively frequent basis and, as a kid, I was the proud owner of an immense whale rib-bone found on the beach in West-Cork and displayed it to anyone who’d let me.
This week, there was a lot of media attention around the discovery of a 5.8 metre giant squid caught in a trawling net 190km off the Kerry coast. Interestingly, the father of the that fishing vessel’s skipper also caught two giant squids off the Kerry coast back in 1995 – the last recorded sighting in Irish waters. In all of Ireland’s documented history, there have in fact been only six sightings of this creature (the first being in 1673, the next in 1875 around Inisbofin and three others in 1995).
The giant squid lives in extremely deep waters (40 or 50 feet) and only rises to shallower levels when chasing fish to feed. They also tend to live quite far offshore (obviously, as this is where the water’s deepest). For this reason, the likelihood of such creatures washing up on the Irish coastline is actually pretty slim even over a prolonged time period. Most of the squid that die probably end up sinking or being eaten. Those that do float are subject to tidal currents and given the great distances from, are likely to hit the Irish coast in very tiny numbers if at all. Even if the body of such a creature did hit the coast, there’s also a low probability it would do so along an accessible part of the shore or on an area frequented by people (the population of early Iron age Ireland, for example was very low compared to today’s populations).
All in all, this probably goes someway to explaining why there are no native Irish stories of sea creatures with distinctive beaks and the ability to release impressive dark clouds of ink.