How can a king lose his throne?

There are many ways a king can lose his throne in medieval ireland, most of which are through the loss of honor.
1. When a poet from the professional class satirizes a king, the poet is telling others how the kings acted in an unjust manner, and therefore is not worthy to rule. The word of a poet is very powerful and may influence others to call for the removal of the king. For more information on poets please visit the poet page here.
2. It is not always necesary for a poet to satirize a king before the king realizes he is unjust or others decide he gives unjust rulings. An example of a King acting unjustly is seen in the liteary story of The Heroic Biography of Cormac Mac Airt, which is about the rise of Cormac, son of Art, to kingship when he replaces the unjust King of Tara. In this story, the current king, Mac Con, rules that a woman must forfeit all of her sheep because they ate expensive shrubs in the Queen's garden. When Cormac hears the ruling, he says that it would be more fair for the sheep to be shaved and the wool given to the Queen instead, "More fitting would be one shearing for another." When Mac Con hears what Cormac has said, he realizes that Cormac would give better judgements and is more worthy of being King of Tara. This story can be found in Tomás Ó Cathasaigh’s book title the Heroic Biography of Cormac Mac Airt (Dublin Institue for Advanced Studies, 1977).
3. Another way a king could lose his honor, and therefore his throne, is when he refused to offer hospitality to visitors. Hospitality was an important aspect in medieval Ireland, and many Law Codes cite the circumstances of how proper hospitality can be measured. The Crith Gablach is a law code translated by Eoin MacNeill in his text Ancient Irish Law: Laws of Satus or Franchise (Proceedings of the Royal Irish Academy, 1923) includes this and other characteristics that a king must have or he loses his honor.
4. Often times the legends surrounding the reign of a new king, incorporate laws the divine say the king must follow. Should the king break these laws during his reign he is breaking his geis, which could result in many terrible events that eventually cause the death of the king. An example of these restrictions, called geis, is seen in the literary story of the Destruction of DaDerga's Hostel, which is about the rise and fall of Conare Mac, King of Ireland. (Early Irish Myths and Sagas, Jeffrey Ganz). In this story Conare Mac must obey many rrestrictions during his reign. Some of the restrictions include; You should not hunt the evil-beasts of Cerna. You should not spend more than nine nights away form Tara. After sunset neither one woman or one man may enter your house. You shall not settle a quarrel between nobles until they come to you for judgement. Conare sequentially breaks all of these geis, which results in his death and loss of the kingship of Ieland. This story is in Early Irish Myths and Sagas translated by Jeffrey Gaz (Penguin Books, 1981), but also can be found as a different translation in this link here.