It was Louis who created the next half chance cutting in and hitting a left footed curler just past the far post.
A low cross from Stevens somehow evaded everyone, including Jimmy Martin arriving at the far post.
Keegan's corner was met at the far post by a Cusack header which beat goalkeeper Alan O'Shea.
Moments later Ritchie Humphreys flicked a header past the far post.
Arsenal broke away to notch their fourth six minutes later before Murtagh cut the deficit back to two with a superb far-post header.
