Idril was born in Aman during the Years of the Trees. She was the daughter and only child of Elenwë and Turgon, who later became king of Gondolin.

After Melkor and Ungoliant had destroyed the Two Trees, Idril was among the Noldor who went to Middle-Earth. Together with many others, she crossed the Helcaraxë, the Grinding Ice, led by Fingolfin and his sons, as well as by Finrod and Galadriel. Many perished in the crossing, among which was Elenwë, Idril’s mother.

Idril lived in Nevrast until Gondolin was built, then she and Turgon moved thither. The Hidden City of Gondolin was fair, and fit to compare even with Elven Tirion beyond the sea.

“But fairer than all the wonders of Gondolin was Idril, Turgon’s daughter, she that was called Celebrindal, the Silver-foot, whose hair was as the gold of Laurelin before the coming of Melkor.” (The Silmarillion)

Idril was called Celebrindal, the Silver-foot, because she always went barefoot. She lived long in bliss, and she received from Enerdhil, a jewel-smith, the Elessar, a green stone, possibly the same one as Aragorn wore during the War of the Ring.

But 200 years after Gondolin was built, Aredhel – the sister of Turgon, left Gondolin. Aredhel officially went to seek Celegorm, one of the sons of Fëanor, who was her friend.
But as is told in the Silmarillion, Aredhel was lost in the wood of Nan Elmoth. There she met Eöl, she married him and bore him a son, Maeglin. Aredhel and Maeglin later came back to Gondolin, closely followed by Eöl. Soon after his arrival, he tried to take back Maeglin, but in the process he accidentally killed Aredhel and because of that, he was cast down from the sheer walls of the city. However, after his mother and father’s deaths, Maeglin stayed in Gondolin, and he came to greatly desire his cousin Idril:

“Yet to none were his [Maeglin’s] eyes more often drawn than to Idril the King’s daughter, who sat beside him; for she was as golden as the Vanyar, her mother’s kindred, and she seemed to him as the sun from which all the King’s hall drew its light.”

Idril however, did not love Maeglin at all, not the least because the Eldar did not wed with such close kin. Years passed while Maeglin continued to watched Idril, and his love turned to darkness in his heart.

After the Nirnaeth Arnoediad, the Fifth Battle of Beleriand, a mortal came to Gondolin. His name was Tuor, and he came with counsel from Ulmo, saying that the Gondolindrim had to abandon their city, and go down Sirion to the Sea. But Turgon had become proud, and he refused to leave Gondolin.
Tuor, however, stayed in Gondolin, and seven years later he married Idril Celebrindal. In the spring of the next year, Idril bore Tuor a son, Eärendil.

In these days, Gondolin was still full of peace and joy, but Idril, who was wise and far-seeing, prepared a secret way out of Gondolin. The work was known but to few, and no whisper of it reached Maeglin’s ears.

The Fall of Gondolin

One time when Maeglin was quarrying after metals in the mountains around Gondolin, he was captured by orcs, and taken to Angband. He eventually revealed the location of Gondolin to Morgoth, who was threatening torture, and who had promised him Idril in return for information.

If it hadn’t been for Idril, there would probably be no survivors when Morgoth attacked Gondolin some years later. She and Tuor led the remnant of the Gondolindrim down the Sirion to Nan-tathren, the Land of the Willows, where the power of Ulmo still ran strong in the river.

Then the sea-longing awoke in Tuor’s heart, and together with Idril and Eärendil they went to the mouths of Sirion and there joined with Elwing’s people. There they dwelt until Tuor felt old age creep upon him. He then built a great ship, Eärrámë, which meant Sea-Wing. Idril and Tuor sailed into the West, and their fate is unknown, but it is said that they came to Eldamar, and Tuor was admitted:

“But in after days it was sung that Tuor alone of mortal Men was numbered among the elder race, and was joined with the Noldor, whom he loved; and his fate is sundered from the fate of Men.”

Research by Celedë_Anthaas

Sources: The Silmarillion, Unfinished Tales