The longest river in Ireland begins in County Cavan. The Shannon Pot, on the slopes of Cuilcagh mountain in the far west of the county, is a small, deep limestone basin in which the river simply appears out of the ground — water that has been working its way underground for kilometres before emerging here to begin a 386-kilometre journey south through nine counties to the Atlantic at Limerick. The River Erne also rises in Cavan, at Beaghy Lough near Stradone, and flows 120 kilometres north into the great Lough Erne complex of Fermanagh. Other rivers — the Blackwater, the Dee, the Annalee, the Cladagh, the Glyde, the Owenroe — all source in Cavan and flow out across Ulster and Leinster. The county is, in a quiet way, one of the great hydrological hubs of Ireland. The Cuilcagh range, just under 665 metres at its highest, is the source of nearly everything that drains out across the surrounding lowlands.

Cavan's medieval history is the history of Breifne — the Gaelic kingdom that occupied this corner of Ireland from roughly the seventh century onwards. By the twelfth century the kingdom had been ruled for generations by the O'Rourke clan from the western half (modern Leitrim) and by the O'Reillys from the eastern half (modern Cavan). Norman pressure in the late 1100s pushed Breifne into a defensive posture: drumlins, lakes, and bad clay soils made the country naturally difficult to invade. Around 1256 the kingdom split formally into West Breifne (O'Rourkes) and East Breifne (O'Reillys). In 1584, under Queen Elizabeth I, the entire area was reorganised — Cavan became a county and was transferred from Connacht to Ulster, where it has remained ever since. The colloquial name "the Breffni County" still appears on Cavan GAA jerseys and in local newspapers. From 1610, under James VI and I, the Plantation of Ulster brought English and Scottish settlers to towns like Bailieborough, Cootehill, Killeshandra and Virginia, and laid down the religious and demographic patterns that still shape the county today.

Cavan suffered terribly during the Great Famine of 1845–1849. The winter of 1847 — Black '47 — brought typhus and cholera on top of starvation, and the population began a long collapse. Mountnugent parish saw one landlord evict more than two hundred people; the local Catholic priest witnessed it and the ballad "By Lough Sheelin Side" preserves the memory. Cavan also produced one of the great Irish poets of the eighteenth century, Cathal Buí Mac Giolla Ghunna — "Yellow Cathal McElgunn" — author of "An Bonnán Buí" ("The Yellow Bittern"), a lament for a bird that drank itself to death on a frozen lake, and an implicit lament for the poet's own drinking too. The hymn "Be Thou My Vision" is sometimes traced back to Saint Dallán Forgaill, a sixth-century Cavan-born poet. The county has had an outsized influence on Irish literature for its size.

Cavan has been an emigrant county for centuries, and some of its emigrants made extraordinary careers abroad. Marcus Daly, born near Ballyjamesduff in 1841, emigrated to America in 1856 and became one of the "Copper Kings" of Butte, Montana — the founder of the Anaconda Copper Mining Company, at one point one of the largest corporations in the world. He built the town of Hamilton, Montana, partly as an act of revenge against a rival. Saint Kilian, born in Mullagh around 640, became a missionary in Würzburg in Bavaria and is still the patron saint of that German city more than 1,300 years later. Thomas "Broken-Hand" Fitzpatrick, born in Cavan in 1799, became one of the great American mountain men and frontier guides of the early nineteenth century, leading the first wagon trains across the Oregon Trail. Patrick O'Rorke, born in Cavan, died as a US Union colonel at Gettysburg in July 1863. The list runs on; the county is small, the diaspora vast.

In modern Cavan, Gaelic football is the dominant sport. Cavan GAA won the All-Ireland Senior Football Championship five times between 1933 and 1952 — including the famous 1947 final played in the Polo Grounds in New York, the only All-Ireland ever played outside Ireland — and although the county has not won the senior title since, it remains one of the most decorated counties in Ulster. The first GAA club in Ulster was founded at Ballyconnell in 1885. Cavan disbanded its senior hurling team in 2011 because the sport had declined too far locally; the team was reformed in 2017 and now plays in Division 3B of the National Hurling League. The county's other great economic story is dairy: Lakeland Dairies, headquartered in Killeshandra, is Ireland's second largest dairy co-operative, with annual revenue exceeding €1.9 billion and farms across the county supplying milk from approximately 219,568 cattle. Lakeland Dairies, the lakes themselves, the drumlins, the GAA, the diaspora — the things that make Cavan recognisable are all variations on the same theme: this is a small place that has held its identity by working at it, generation after generation.