Ruadán was known for his prophecies. His prophecy that Diarmait mac Cerbaill would die from the roof beam of his house came true, even though Diarmait had it removed and cast into the sea: it was found and innocently used as a roof beam in another house, in which Diarmait was trapped when it was burned down. The roof beam—his own—fell on him.
Ruadán was not just objectively prophesying Diarmait's death: he actively disapproved of Diarmait. Diarmait had imprisoned a kinsman of Ruadán who had violated the king's law. Ruadán came to Tara to curse Diarmait for violating he sanctity of the church; he cursed the hearth, proclaiming that never again would smoke rise from a building on the Hill of Tara. Diarmait then glanced up at the ceiling, at which Ruadán made his famous prediction about the roof beam.
The Curse of Tara and the "decommissioning" of the royal hill site would therefore have started as of Ruadán's visit in 556. Diarmait survived until 565, however, and Adomnán of Iona held a synod at Tara in 697.
Ruadán died in 584; his feast day is 15 April. There is not a lot of detail about his life, but his reputation lived on: he "appeared" in the 12th century to an Irish knight named Tnugdalus (alternately, Tundalus or Tundale). The popular Visio Tnugdali ("Vision of Tnugdalus") was recorded about 1149 by a Brother Marcus who claimed to have heard it from the knight himself.
But that's a story for tomorrow.