Lyttelton, National THEATRE, LONDONYellow Face The Shed, National Theatre, LondonREGARDLESS OF the quality of subject matter or dialogue, plays can easily be scuppered by having the wrong structure: length or sequence of scenes, even the positioning of an interval. So it’s intriguing that the National Theatre’s early summer schedule includes two peculiarly shaped scripts of different vintages.The Silver Tassie by Seán O’Casey has the distinction of being considered simultaneously a classic of Irish literature and a bit of a dog’s dinner dramatically. The play was turned down, in 1928, by the Abbey Theatre in Dublin largely because it was thought theatrically incoherent. Against all the rules of dramaturgy and logic of economics, each of the four acts takes p