Sir Liam Ó Seachnasaigh (died 1579) was an Irish Chief of the Name.
Ó Seachnasaigh was the eldest legitimate son of Sir Ruaidhrí Gilla Dubh Ó Seachnasaigh by Lady Honora O'Brien, daughter of Murrough O'Brien, 1st Earl of Thomond, but till 1573 had been usurped as Chief by his uncle, Diarmaid Riabach.
Diarmaid was deposed but in 1579 ambushed Sir Liam at Ard Maoldubhain.
Sir Liam was succeeded by his younger brother, Dermot.
This Irish biographical article is a stub.