The King's Mirror (Speculum Regale—Konungs Skuggsjá).
Translated from the Old Norse with Introduction and Notes by Laurence Marcellus Larson (New York: Twayne Publishers, Inc., & The American-Scandinavian Foundation, 1917), p. 118 (with translator's note; on Ireland):
Long time ago a clownish fellow lived in that country; he was a Christian, however, and his name was Klefsan.* It is told of this one that there never was a man who, when he saw Klefsan, was not compelled to laugh at his amusing and absurd remarks. Even though a man was heavy at heart, he could not restrain his laughter, we are told, when he heard that man talk. But Klefsan fell ill and died and was buried in the churchyard like other men. He lay long in the earth until the flesh had decayed from his bones, and his bones, too, were largely crumbled. Then it came to pass that other corpses were buried in the same churchyard, and graves were dug so near the place where Klefsan lay that his skull was unearthed, and it was whole. They set it up on a high rock in the churchyard, where it has remained ever since. But whoever comes to that place and sees that skull and looks into the opening where the mouth and tongue once were immediately begins to laugh, even though he were in a sorrowful mood before he caught sight of that skull. Thus his dead bones make almost as many people laugh as he himself did when alive.
Aislinge Meic Conglinne. The Vision of MacConglinne. A Middle-Irish Wonder Tale.
* A somewhat different version of this tale is found in the poem on the "Wonders of Ireland" (Reliquiae Antiquae, II, 105). See also Ériu, IV, 14.
Edited with a Translation (Based on W.M. Hennessy's), Notes, and a Glossary by Kuno Meyer (London: David Nutt, 1892), p. 131 (editor's note):
Mac Rustaing, according to a note in the LBr. commentary on the Félire (Stokes' ed., p. cxlv), was a brother of St. Coemán Brecc. But this cannot have been the case, for Coemán died in 615. In the same note it is stated that Mac Rustaing lies buried at Ross Ech (now Russagh, near the village of Street, in the north of co. West Meath), and that no woman can look at his grave without breaking wind or uttering a loud foolish laugh. This is also mentioned as one of the wonders of Erin in Todd's Irish Nennius, p. 201, and a similar story is told in the Old-Norwegian Speculum Regale about the skull of an Irish jester called Clefsan. It would seem, then, that Mac Rustaing was a famous jester in his time.