"the distant rattling of a corncrake"
Corncrake
Creative Commons AttributionCorncrake - Credit: Sergey Yeliseev

The corncrake (Crex crex) is a shy, secretive bird.  It is endangered in Ireland, and is probably not actually seen or heard in the area in which The Sea takes place.   

The corncrake makes a noise like 'two cheese graters rubbed together, producing a sound so monotonous as to qualify the bird as the world's worst singer' -  BirdWatch Ireland.

Listen on the RSPB Website

It is perhaps no coincidence that Banville chose the corncrake, for the young Max is also shy and secretive in his obsession with the Graces.