This narrative poem is composed in three parts, and consists of a dialogue between the aged Irish hero Oisín and St. Patrick.
Oisín relates his three-hundred year sojourn in the immortal isles of Faerie. In the isles, Oisín married the beautiful Sidhe Niamh: together they traveled, feasted, and quested. At last Oisín succumbs to the temptation to return and visit the lands of mortal men: inadvertently slipping from his faerie horse, his body touches the ground and instantly becomes that of a decrepit old man.
Oisín describes his travels and adventures in various of the immortal isles: contrasting his noble past deeds with the present degenerate weakness of the current generation.