Yes Iravan was a real character in the Mahabharat but there are many legends associated with him that do not find mention in the scriptures.According to the original scripture he was the son of Arjun from the Naga princess Ulupi whom he met while he was on exile.
Chapter 91 of the Bhishma Parva of Mahabharat mentions the story of Iravan as follows:
This handsome and valiant son of Arjuna, named Iravat, was begotten upon the daughter of the king of the Nagas by the intelligent Partha. Her husband having been slain by Garuda, she became helpless, and of cheerless soul. Childless as she was, she was bestowed (upon Arjuna) by the high-souled Airavat. Partha accepted her for wife, coming to him as she did under the influence of desire. It was thus that that son of Arjuna was begotten upon the wife of another.
Abandoned by his wicked uncle from hatred of Partha, he grew up in the region of the Nagas, protected by his mother. And he was handsome and endued with great strength, possessed of diverse accomplishments, and of prowess incapable of being baffled.
Arjun comes to know of his existence when he meets him in the court of his father Indra:
Hearing that Arjuna had gone to the region of Indra, he speedily went thither. And the mighty-armed Iravat, possessed of prowess incapable of being baffled, approaching his sire, saluted him duly, standing before him with joined hands. And he introduced himself to the high-souled Arjuna, saying, 'I am Iravat. blessed be thou, and I am thy son, O lord'. And he reminded Arjuna of all the circumstances connected with the latter's meeting with his mother. And thereupon the son of Pandu recollected all those circumstances exactly as they happened. Embracing his son then who resembled himself in accomplishments, Partha, in Indra's abode, was filled with joy.
Realizing that he could be really useful in the inevitable battle that was to hapen in the future, Arjun had then requested him to join forces when the time comes:
The mighty-armed Iravat then, O king, in the celestial regions was, O Bharata, joyfully commanded by Arjuna, with regard to his own business, (in these words), 'When the battle takes place, assistance should be rendered by thee'. Saying 'Yes', O lord, he went away. And now at the time of battle he presented himself. O king, accompanied with a large number of steeds of great fleetness and beautiful colour.
The same chapter also mentions his bravery in the war and what happened to him on the 8th day of battle:
Those six brothers (of Shakuni) endued with great strength, viz., Gaya, Gavaksha, Vrishava, Charmavat, Arjava, and Suka dashed out of the mighty (Kaurava) array, supported by Sakuni and by their respective forces of great valour, themselves clad in mail, skilled in battle, fierce in mien, and possessed of exceeding might. Breaking through that invincible cavalry division (of the Pandavas), O thou of mighty arms, those Gandhara warriors who could with difficulty be vanquished, supported by a large force, desirous of heaven, longing for victory, and filled with delight, penetrated into it. Beholding them filled with joy, the valiant Iravat, addressing his own warriors decked with diverse ornaments and weapons, said unto them, 'Adopt such contrivances in consequence of which these Dhritarashtra warriors with their weapons and animals may all be destroyed.' Saying 'Yes', all those warriors of Iravat began to slay those mighty and invincible Dhartarashtra soldiers.
What happened next is actually reminiscent of the Abhimanyu episode when the six brothers decide to attack Iravan together:
Beholding that their own warriors were thus overthrown by Iravat's division, those sons of Suvala being unable to beat it coolly, all rushed at Iravat and surrounded him on all sides. And commanding (all their followers) to attack those of Iravat with lances, those heroes swept over the field, creating a great confusion. And Iravat, pierced with lances by those high-souled warriors, and bathed in blood that trickled down (his wounds), looked like an elephant pierced with the hook. Wounded deeply on the chest, back, and flanks, singly encountering the many, he did not yet, O king, swerve from his (natural) firmness. Indeed, Iravat, excited with rage, deprived all those adversaries of their senses, piercing them, in that battle, with sharp shafts. And that chastiser of foes, tearing those lances from off his body, struck with them the sons of Suvala in battle.
Here the ending takes a turn - While Abhimanyu had succumbed to the combined onslaught of the cowards who had surrounded him Iravan managed to kill all of them except one who escaped.
Then unsheathing his polished sword and taking a shield, he rushed on foot, desirous of slaying Suvala's sons in that combat. The sons of Suvala, however, recovering their senses, once more rushed at Iravat, excited with wrath. Iravat, however, proud of his might, and displaying his lightness of hand, proceeded towards all of them, armed with his sword. Moving as he did with great activity, the sons of Suvala, although they moved about on their fleet steeds, could not find an opportunity for striking that hero (on foot). Beholding him then on foot, his foes surrounded him closely and wished to take him captive. Then that crusher of foes, seeing them contiguous to himself, struck off, with his sword, both their right and left arms, and mangled their other limbs. Then those arms of theirs adorned with gold, and their weapons, fell down on the earth, and they themselves, with limbs mangled, fell down on the field, deprived of life. Only Vrishava, O king, with many wounds on his person, escaped (with life) from that dreadful battle destructive of heroes.
Of course this could not have gone unnoticed by Duryodhan who then ordered the rakshas Alambusha to attack Iravan. Since both Iravan and Alambusha were master of illusion the next steps of the battle are actually quite interesting:
That slayer of foes, viz., the valiant Iravat, excited with rage, and advancing speedily from desire of slaying the Rakshasa, began to resist him. Beholding him advance, the mighty Rakshasa speedily set himself about for displaying his powers of illusion. The Rakshasa then created a number of illusive chargers which were riden by terrible Rakshasas armed with spears and axes. Those two thousand accomplished smiters advancing with rage, were however, soon sent to the regions of Yama, (falling in the encounter with Iravat's forces). And when the forces of both perished, both of them, invincible in battle, encountered each other like Vritra and Vasava. Beholding the Rakshasa, who was difficult of being vanquished in battle, advancing towards him, the mighty Iravat, excited with rage, began to check his onset. And when the Rakshasa approached him nearer, Iravat with his sword quickly cut off his bow, as also each of his shafts into five fragments. Seeing his bow cut off, the Rakshasa speedily rose up into the welkin, confounding with his illusion the enraged Iravat. Then Iravat also, difficult of approach, capable of assuming any form at will, and having a knowledge of what are the vital limbs of the body, rising up into the welkin, and confounding with his illusion the Rakshasa began to cut off the latter's limbs in that battle and thus were the limbs of the Rakshasa repeatedly cut into several pieces.
So Iravan again emerges victorious but that did not mean a happy ending for him. The demon actually re-emerged from his mangled body to overpower Iravan:
Then (Alamvusha), the mighty son of Rishyasringa, beholding his foe blazing forth with energy, became infuriate with rage and himself put forth his prowess in that combat. Assuming a prodigious and fierce form, he endeavoured to seize the heroic son of Arjuna, viz., the renowned Iravat. In the sight of all the combatants there present, beholding that illusion of the wicked Rakshasa in the van of battle, Iravat became inflamed with rage and adopted steps for himself having recourse to illusion. And when that hero, never retreating from battle, became inflamed with wrath, a Naga related to him by his mother's side, came to him. Surrounded on all sides, in that battle by Nagas, that Naga, O king, assumed a huge form mighty as Ananta himself. With diverse kinds of Nagas then he covered the Rakshasa. While being covered by those Nagas, that bull among Rakshasas reflected for a moment, and assuming the form of Garuda, he devoured those snakes. When that Naga of his mother's line was devoured through illusion, Iravat became confounded. And while in that state, the Rakshasa slew him with his sword, Alamvusha felled on the earth Iravat's head decked with ear-rings and graced with a diadem and looking beautiful like a lotus or the moon.
As you can see in the original epic Iravan gets killed by Alambusha in the battle on the 8th day of Mahabharat and not as a sacrifice to Goddess Kali as mentioned in the story of Krishna turning into Mohini and marrying Iravan. It was not a pre-meditated plan to kill him to please any goddess and in fact the Pandavs only later find out about his death and it is Bhima who breaks the news to Arjun as mentioned in Chapter 96:
Then commenced a fierce battle between Bhagadatta, O sire, and the Panchalas, the Srinjayas, and the Kekayas, with upraised weapons. Then Bhimasena, in that battle told both Kesava and Arjuna in detail about the slaughter of Iravat as it had occurred."
