Athirne Ailgheasach ("the importunate"), son of Ferchertne, is a poet and satirist of the court of Conchobar mac Nessa in the Ulster Cycle of Irish mythology, who abuses the privileges of poets.
He stole three cranes from Midir of the Tuatha Dé Danann, which stand at his door and refuse entry or hospitality to anyone who approaches.
In the saga "The Siege of Howth", he goes on a circuit of Ireland, visiting kings' courts, and making outrageous demands of hospitality, knowing that disgrace would fall on any kingdom that refused him, and that if anything happened to him the Ulstermen are bound to go to war in his defence. He demands the remaining eye of the one-eyed king of southern Connacht, Eochaid mac Luchta, a night with the wives of Tigerna Tétbuillech, king of Munster and Mesgegra, king of Leinster, and a mysterious jewel from another Leinster king, Fergus Fairge, which Fergus only finds by appealing for help to the "Lord of the Elements". He captures 150 wives of Leinster nobles and prepares to take them back to Ulster with him. The men of Leinster pursue him, the men of Ulster come to his defence, and battle is joined. The Ulstermen are besieged in the fort of Howth Head, north of Dublin, but break out, and the Leinstermen are put to flight. In the ensuing pursuit, the Ulster hero Conall Cernach kills Mesgegra in single combat and takes his head.
When Amergin, son of Eccet Sálach the smith, who has lived to the age of fourteen without speaking, suddenly utters a cryptic poem, Athirne fears the boy will replace him as chief poet of Ulster, and resolves to kill him with an axe. Eccet foils the murder attempt by making a lifelike replica of the boy from clay. The Ulstermen besiege Athirne in his house and force him to pay compensation to Eccet. He takes Amergin as his foster-son and trains him as a poet.