Seán Ó Néill, anglicized Shane Ó Neill (c. 1530 - 2 June 1567), nicknamed 'Seán an díomais', is an Irish chief of the Ó Néill clan of Ulster in the mid 16th century.

Shane O'Neill's career is marked by his ambition to be The Ó Néill Mór - chief of all the O'Neills.

This brings him into conflict with competing branches of the O'Neill family and with the English government in Ireland, who recognise a rival claim.

Shane's support is considered worth gaining by the English even during the lifetime of his father Conn O'Neill, 1st Earl of Tyrone (died 1559).

But rejecting overtures from Thomas Radclyffe, 3rd Earl of Sussex, the lord deputy from 1556, Shane refuses to help the English against the Scottish settlers on the coast of Antrim, allying himself instead with the MacDonnells, the most powerful of these immigrants.