Seamus and Dean were among the five boys sorted into Gryffindor in 1991, the other three being Harry Potter, Ronald Weasley and Neville Longbottom. Seamus and Dean became best friends in their first year and remained friends for the rest of their time at Hogwarts.
In the summer of 1994, Dean attended the Quidditch World Cup with Seamus and his mother. The pair supported Ireland as Seamus was Irish.
In their fifth year, Seamus did not believe Harry's claims of Voldemort's return while Dean did, but this did not affect the two boys' friendship. Dean eventually managed to get Seamus to join Dumbledore's Army, although the latter only managed to attend one meeting as the group was exposed.
As Dean was unable to attend Hogwarts in 1997-1998 due to him being a suspected Muggle-born, Seamus spent most of his seventh year missing Dean. During the Battle of Hogwarts, past members of Dumbledore's Army returned to Hogwarts to fight, and Dean was among them. Seamus ran to hug Dean when he saw him. The two survived the Battle of Hogwarts, and sat with Aberforth Dumbledore after the battle was over.
Not much is known about their life after the battle. It is possible that they attended 2014 Quidditch World along with other members of Dumbledore's Army.
Deamus is one of the more common slash pairings. On AO3, it is the most written ship for both Dean and Seamus. It is also the 22nd most written pairing within the Harry Potter tag.
- Both of the actors of Dean and Seamus were supportive of the ship with Devon Murray tweeting that they could now get married when same-sex married was approved. 
- Devon Murray tweet (May 2015)
|SHIPS||femslash||Bellamione • Linny • Parkwood • TNT|
|slash||Deamus • Drarry • Flintwood • Grindeldore|
Scorbus • Starbucks • Tomarry • Wolfstar
|friendships||Golden Trio • Marauders|
|het||Bellamort • Billdora • Bleur • Charry • Chedric • Dramione • Drastoria • Druna • Fremione • Georgelina • Harmony • Hinny • Huna • Jakweenie • Jily • Lovander • Marthur • Nadence • Nevannah • Newta • Newtina • Nuna • Perdrey • Ravender • Red Moon • Remadora • Romione • Snily • Tedoire • Tedromeda • Theta|
|non-binary||Barnaby x MC • Bill x MC • Charlie x MC • Chiara x MC|
Felix x MC • Penny x MC • Rowan x MC • Talbott x MC
|family||Black Sisters • Severitus • Weasley Family|
|CHARACTERS||m/f||Harry Potter • Hermione Granger • Jacob's sibling|