The river represents a major physical barrier between east and west, with fewer than thirty-five crossing points between the village of Dowra in the north and Limerick city in the south.

[n 1] Further sinks that source the pot include Pollboy and, through Shannon Cave, Pollahune in Cavan and Polltullyard and Tullynakeeragh in County Fermanagh, Northern Ireland.

Settlements along the river (going upriver) include Kilrush, Tarbert, Glin, Foynes, Askeaton, Shannon Town, Limerick, Castletroy, Castleconnell, O'Briensbridge, Montpelier, Killaloe, Ballina, Portumna, Banagher, Athlone, Lanesborough, Carrick-on-Shannon, Leitrim village and Dowra.

Ptolemy's Geography (2nd century AD) described a river called Σηνος (Sēnos) from PIE *sai-/sei- 'to bind', the root of English sinew and Irish sin ‘collar’, referring to the long and sinuous estuary leading up to Limerick.

In the 17th century, the Shannon was of major strategic importance in military campaigns in Ireland, as it formed a physical boundary between the east and west of the country.

In preparing a land settlement, or plantation after his conquest of Ireland Oliver Cromwell reputedly said the remaining Irish landowners would go to "Hell or Connacht", referring to their choice of forced migration west across the river Shannon, or death, thus freeing up the eastern landholdings for the incoming English settlers.

William Ockenden, also from England, was placed in charge of works on this stretch in 1757 and spent £12,000 over the next four years, without fully completing the task.

In 1791, William Chapman was brought in to advise and discovered a sorry state of affairs – all the locks had been built to different dimensions and he spent the next three years supervising the rebuilding of most of them.

But even then, there were no tow paths in the river sections and there were still shoals in the summer months, as well as a lack of harbour facilities at Limerick, and boats were limited to 15–20 long tons (15–20 tonnes) load, often less.

After protracted negotiations on costs and conditions, the work was completed by 1810, so that boats drawing 1.75 m (5 ft 9 in) could pass from Athlone to Killaloe.

[citation needed] In the latter part of the 1820s, trade increased dramatically with the arrival of paddle-wheeled steamers on the river which carried passengers and goods.

The old Killaloe to Limerick canal with its five locks was abandoned and the head race constructed from Lough Derg also served for navigation.

Near Limerick, a short canal connects Plassey with the Abbey River, allowing boats to bypass the Curraghower Falls, a major obstacle to navigation.

In some sources she, like Fionn mac Cumhaill, caught and ate the Salmon of Wisdom who swam there, becoming the wisest being on Earth, in others, she merely drank from the well.

Patricia Monaghan notes that "The drowning of a goddess in a river is common in Irish mythology and typically represents the dissolving of her divine power into the water, which then gives life to the land".

[30] A small myth about Sionann tells that the legendary hunter-warrior Fionn mac Cumhaill was attacked by a number of other warriors at Ballyleague, near north Lough Ree.

It further says that Fionn was afraid of the power of the stone and threw it into the river, where it remains at a low ford, and that if a woman named Be Thuinne finds it, then the world's end is near.

[29] The Shannon reputedly hosts a river monster named Cata, the first known mention being in the medieval Book of Lismore.

In this manuscript, we are told that Senán, patron saint of County Clare, defeated the monster at Inis Cathaigh.

[31] Cata is described as a large creature with a horse's mane, gleaming eyes, thick feet, nails of iron, and a whale's tail.

[32] Another story has an oilliphéist flee its home in the Shannon, upon hearing that Saint Patrick has arrived to remove its kind from Ireland.

[33] Despite being 360.5 km (224.0 mi) long, it rises only 76 m (249 ft) above sea level, so the river is easily navigable, with only a few locks along its length.

60,000-tonne cargo vessels now carry raw bauxite from West African mines to the plant, where it is refined to alumina.

Water would be pumped to a break pressure tank Knockanacree near Cloughjordan in County Tipperary and gravity fed from there by pipeline to Dublin.