In J. R. R. Tolkien's universe of Middle-earth, Cirion, son of Boromir I, was the twelfth ruling Steward of Gondor.

During his rule, the evil Men known as Balchoth gathered for an assault upon Gondor, passing over the Undeeps of Anduin into the northern province of Calenardhon. Cirion could not spare many men to defend this part of his realm, and in despair he sent several messengers to their old friends the Éothéod, a people living in the far north.

Only one man, Borondir, reached the Éothéod capital of Framsburg, and deliverd his message to the king Eorl the Young.

After not receiving any answer for a full year, Cirion set out west through Anórien, to prevent the Balchoth from crossing the Anduin at the Undeeps and the Wold.

The army of Gondor was defeated and driven back across the river Limlight to the Field of Celebrant. In what was probably a coordinated attack, Orcs had come out of the Hithaeglir (Misty Mountains), and assailed the flank from the west, driving the forces of Gondor towards the Anduin. All hope was lost, when unlooked for came the riders of Éothéod out of the North. Together the Éothéod and the army of Gondor defeated the Balchoth and Orcs, driving them into the Wold.

Cirion was a wise man, and saw that he could use the Éothéod further. He told Eorl that he would leave Calenardhon in his care, and meet Eorl again in three months' time. Back at Minas Tirith, Cirion ordered some of his most trusted servants to go to the Firien Woods in Anórien, and in secret clear the way to the mountain of Amon Anwar at the Halifirien beacon.

Three months later Cirion, his son Hallas, the Prince of Dol Amroth, and two others Councilors of Gondor set out for Calenardhon, and Cirion met with Eorl. They ascended Amon Anwar, and Cirion revealed the shrine to Elendil, built in secret at this mountain, as it formed the centre of the Gondor of old. At this secret place, he revealed his plan to give Calenardhon as a free gift to the Éothéod, so that there should be a perpetual friendship between the Éothéod and the people of Gondor. On the behalf of his people, Eorl accepted this deal, swearing the Oath of Eorl and becoming the first King of Rohan.