Title: The Iron Shirts
Subtitle: A Tor.Com Original
Description: In a world in which horses survived to flourish in 13th-century America, the wars and rivalries of thirteenth-century Ireland have new players: the mysterious warriors from across the Western Sea, called by some the "ó Gonklins." They and their Irish hosts have much in common—enough to misunderstand one another far more than utter aliens ever could.

([Source][1])

[1]: https://us.macmillan.com/books/9781429930178/theironshirts/
Subjects: Science Fiction, Alternate History, Short Stories, Fiction, science fiction, general