Melaghlin Reagh Ua Seachnasaigh (died 1179) was an Irish Chief of the Name.
Ó Seachnasaigh was the first bearer of the surname to be listed as of Cenél Áeda na hEchtge. However, the territory seems to have been contended with the Ó Cathail family. The Annals of the Four Masters record, sub anno 1179:
Melaghlin Reagh O'Shaughnessy, Lord of half the territory of Kinelea, was killed by the son of Donough O'Cahill.
He was a descendant of Seachnasach mac Donnchadh.
References
http://www.ucc.ie/celt/published/T100005C/ John O'Donovan. The Genealogies, Tribes, and Customs of Hy-Fiachrach. Dublin: Irish Archaeological Society. 1844. Pedigree of O'Shaughnessy: pp. 372–91. Galway: History and Society, 1996