The protagonist is Conn the Thrall, who fights alongside Turlogh Dubh O'Brien, a recurring character of Howard's who is an outcast from Brian Boru's clan.
He then added fantasy elements to the story so as to be able to submit it to the Weird Tales magazine under the title The Grey God Passes, where it was also rejected by editor Farnsworth Wright in December, 1931.
With the victory of Christian King Brian over the enslaving Vikings, at the cost of their own lives, Ireland is freed from bondage and increasingly left in the hands of former slaves.
(Compare the view of the supernatural in the Kull or Conan the Barbarian stories, with that in the Solomon Kane tales or in The Black Stone or The Children of the Night for instance.)
The notable exception being Worms of the Earth where, in the fourth or fifth century, even a hard bitten hero like Bran Mak Morn finds horror beyond his ability to stand.