1) A door-post in many parts of England (OED) but that is rarely the meaning in Yorkshire.
For example, in a deed of 1549-50, William Lockwood and his wife, of Linthwaite Hall, were held responsible for making the west derne of lane yate and all the fenses betwixt the derne and one close called Ridynges. The ‘yate’ was a gate and the ‘derne’ was therefore a gatepost. A second reference from that period has the same meaning: 1555 from the east end of the yate dearne to the east syde of the mystall, Thurstonland. In Rastrick the term was used in connection with the gate of the town pinfold: 1609 make for the kings pound in Rastrick a yate with sufficiente dearns and a good locke and keye of the townes charge. These ‘dearns’ will have been made of wood, especially since the Rastrick overseers were ordered to maintain the rales and fence of the pinfold. Occasionally, a dearn could be a wooden pillar: in 1627 a stoop or dearne of wood was anciently sett in the ground close by … to distinguish this [dike] from other little rivers, Lingards. Quite frequently ‘dearns’ were referred to in pairs: 1681 A yate att the end of the greate dole adjoyneing to the westfeild with two dearnes to hange itt to come and goe, Kirkheaton. Occasionally, the spelling was closer to examples quoted in the OED, and the meaning could be door-post: 1603 the dore dearne, Netherthong
1671 two new Dornes [sic] for the Doore into the vault, Elland. The word is possibly the source of the Lindley place-name Dearne Fold, for which spellings survive only from the seventeenth century: 1667 William Morehous de Dearne de Lynley
1701 All that messuage called by the name of the Dearne and all houses, edifices, buildings, folds … adjoining to the said house, Lindley. The property formed part of the Thornhill estate, and alternative spellings in estate rentals include Durn and Dirn,/i>. It is a word seldom heard now but it was formerly in widespread use by dialect speakers.