For seven hundred years from the 8th century, they lived in and ruled an area roughly conterminous with present-day south County Leitrim.

The tuath of the Muintir Eolais collapsed with Irish defeat in the Nine Years' War, and became largely forgotten with the English occupation of Ireland.

[8] The entries in the Annals of Lough Ce for 1245 and 1247 suggest a decisive defeat of Mac Costello (also called De Angulo or Nangle), and halted Norman claims to the territory of Muintir Eolais (until 1551; see below).

Specifically, on 5 November 1530, the Muintir Eolais signed an agreement to pay rent to Gerald Fitzgerald, 9th Earl of Kildare, in return for military protection.

It was the aforesaid chief men who dictated the agreement, and Mailin-oge Mac Mailin O'Mailconery, wrote it in their presence, on the fifth day of the month of November, at Maynooth.

The Earl subjects to a penalty of three marks any one who is indebted who shall refuse a pledge to the steward, to wit Concobhair Mac Culruadh: one-half to Magradhnaill and the chief men who made this covenant, and the other half to the Earl.However, by 1533, Henry VIII of England wanted the Englishman, William Skeffington, as replacement Lord of Ireland.

The Annals of the Four Masters states "1540: the castle of Leitrim was erected by O'Rourke (Brian, the son of Owen) while a great war was waged against him on every side, namely, in Moylurg, Muintir-Eolais, and Breifny-O'Reilly".

In 1551, Thomas Nangle the baron of Navan made submissions to the English council of Ireland that Mag Raghnaill was refusing to pay him 100 kine yearly, plus knights fees, due to his ancestors.

[9] On 5 December 1552, the Muintir Eolais, with the approval of the monasteries of Conmaiche, signed a legally binding document, written in neat Irish, deeding the title of chieftain and protector of Muintir Eolais to Sean ("Shane") Reynolds of Clonduff in County Offaly, on condition he lobby the English on their behalf and protect their ancestral lands.

In 1590, an "immense" English army invaded south county Leitrim during the nine years war, which ended in defeat for gaelic Ireland.

Thereafter, the tuath ('nation') of Muintir Eolais was extinguished, and the formation of county Leitrim marked the onset of an English occupation lasting over three hundred years.

[16][22][23] From the end of the Nine Years' War up until 1729 the Reynolds dynasty of Lough Scur owned large estates in south Leitrim.

[25] He maintained an army of "two hundred men", and his reputation for jailing rent defaulters, and beheading people for minor offences, was widely feared.

[26] Folklore claims Sean (or Seóin) was killed by a soldier from Longford avenging his sister's death on Prison Island.

[29][full citation needed] His grandson Sean was captured during by the 1641 Rebellion, held captive by rebels, tortured and probably executed.

[30] Tradition recalls this Sean Reynolds of Lough Scur invited the other Muintir Eolais chieftains to his castle for a reconciliation meeting before, in cruel betrayal, beheading them all.

The names of families descended from the Muintir Eolais are common today: Reynolds, Mulvey, McGarry, Shanley and Moran.

Today at Clonmacnoise monastery a carved headstone is dedicated to Ódhrán Ua hEolais (d. 994), scribe of Clonmacnois, the inscription reading 'Pray for Odhrán descendant of Eolas'.

[citation needed] Parts of Caisleán Seóin at Lough Scur collapsed circa 1908, but repairs were undertaken by a heritage preservation society.

[33][34] In the remote mountainous Cuilcagh-Anierin uplands, the oligotrophic lake named "Lough Munter Eolas" marks a borderline between west Cavan and south Leitrim.

[37] The fictional land of "Clan Eolais" populated by "Eolaisans" and Sylphs, appears in the Solas 2 role playing game.