In Irish mythology and pseudohistory, Diarmaid mac Cearbhaill (Diarmait mac Cerbaill, Diarmuid mac Cerrbeóil, Dermot McKerval) of the southern Ui Néill clan was High King of Ireland ca. 545-561. He was reportedly the last pagan king of Ireland, and the last to celebrate the ritual of the Feis Temrach (the "spending the night" of Tara) in which the new king slept with the local goddess of sovereignty.

Irish annals record that Diarmaid was killed in 561 in the Battle of Cúl Dremne against the northern Uí Néill. A legend records another tradition about his death.

Diarmaid had killed his wife's lover Flann mac Díma by burning down his fortress. For this deed, according to the legend, St. Ronan prophesied that Diarmaid would die when a roofbeam fell on his head; Saint Ciaran prophesied that he would die in the same manner as Flann; but Bec mac Dé, Diarmaid's druid, gave a much more elaborate prophesy, foretelling a threefold death. He would be drowned, burned, and have his head crushed by a roofbeam, by Flann's kinsman Áed Dub , king of Ulster, in the house of Banbán the hospitaller, on a night that he wore a shirt grown from a single flax seed, drank ale brewed from a single grain of corn, and ate pork from a sow that had never farrowed.

Diarmaid dismissed these prophesies, even when invited to a feast by Banbán. His wife, who had been listening, refused to go, so Banbán offered the king his own daughter for that night, and gave him a nightshirt, pork and ale. When the girl told him the nightshirt had been woven from flax grown from a single seed, the ale had been brewed from a single grain of corn, and the pork had come from a sow that had never farrowed, Diarmaid realised the prophesy had been fulfilled, but too late. He tried to escape, but Áed Dubh was standing at the door with his spear. As Diarmaid fled back into the house, Áed's men set it on fire. Diarmaid tried to escape the conflagration by leaping into a vat of ale, but a flaming roofbeam fell on his head, and the prophesy was fulfilled.

Family tree

                Conn Cétchathach
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                 Art mac Cuinn
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                Cormac mac Airt
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               Cairbre Lifechair
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               Fiachu Sraibtine 
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               Muiredach Tirech
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               Eochaid Mugmedon
          + Mongfind        + Cairenn 
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  Brion  Fiachrae  Ailill     Niall
      (The Connachta)           |
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      |          |       |      .      |           |        |             |            |
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Conall Gulban  Endae   Eogan    .   Coirpre    Lóegaire  Maine  Conall Cremthainne  Fiachu
                         |      .      |           |              ________|________
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                    Muirdeach   . Cormac Caech  Lughaid     Fergus Cerrbel      Ardgal
                         |      .      |        (d.507)           |
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                  Muirchertach  .   Tuathal                   Diarmaid
                    mac Ercae   .   Maelgarb               mac Cearbhaill
                     (d.536)    .   (d.544)                   (d.565)
                                .
      (Northern Ui Néill)       .                 (Southern Ui Néill)