This episode begins with a cupcake cake reading "Happy 99th" on the victim's table of baked goods. This is the 99th episode of Castle.
Title reference: "The Wild Rover" is a popular English-language folk song that has become most associated with Irish culture, and has been covered by numerous Irish artists.
Esposito has trouble pronouncing the name Siobhan because of Gaelic spelling rules. This is an inside joke as actor Seamus Dever has a similarly peculiar Irish first name.
When Ryan explains that he worked undercover in Narcotics before transferring to Homicide, he calls the situation complicated, to which Castle replies "I love complicated". In other episodes, "complicated" is not only how he and Beckett describe their romantic relationship, but for most of season four, it's how they both justify their individual reluctances in pursuing it. The adjective is also how Emma, Stana Katic's character in Truth About Kerry (2010) (which happens to be set in Ireland), describes her relationship with her fiancè.
Detective Ryan goes undercover in an Irish gang. Seamus Dever, who plays Detective Ryan, is of Irish descent and has posted pictures on Instagram of himself and family in the "ole country," though he himself was born in Flint, Michigan and raised in Arizona.