Seamus and Dean were among the five boys sorted into Gryffindor in 1991, the other three being Harry Potter, Ronald Weasley and Neville Longbottom. Dean shared a dormitory with Seamus, who soon became his best friend. They remained friends for the rest of their time at Hogwarts. They spent almost all their time together on the school grounds. They also sat next to each other after the Sorting in their first year, at the Gryffindor table. Dean was almost a head taller than Seamus, which Aberforth made a comment about later on.
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. Seamus only started to believe Harry after he read Harry's interview with Rita Skeeter. They attended the Yule Ball separately. 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. Seamus was also displeased when Dean was chosen as the replacement Chaser for the Gryffindor Quidditch team over him when Katie Bell was injured in their sixth year, but again, the two still remained best friends. At the end of the year, Dean attended Dumbledore's funeral with Seamus, after Seamus had rowed rather publicly with his mother to allow him to stay for the ceremony. This would be the last year Dean attended education at Hogwarts School of Witchcraft and Wizardry as the following year he would not return for his own safety.
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, despite not having a wand. Seamus was ecstatic to see him, and ran to hug 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.
- Dean – “We're fighting, aren't we? The message said Harry was back, and we were going to fight! I'll have to get a wand though-”
- Seamus – “You haven't got a wand-?”
- — Harry Potter and the Deathly Hallows: Part 2
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. As of April 2022, there were 2180 works tagged Seamus Finnigan/Dean Thomas on AO3, with 686 using "otp:true". The ship is a popular background pairing, but also has its own dedicated fans.
- Deamus posts on Tumblr
- Dean x Seamus posts on Tumblr
- Dean Thomas x Seamus Finnigan posts on Tumblr
- 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||het||Bellamort • Billdora • Charry • Georgelina • Harmony • Hinny|
Huna • Jakweenie • Jily • Lovander • Marthur • Nadence
Newta • Newtina • Perdrey • Ravender • Red Moon • Remadora • Romione • Scabmione • Scorose • Snily
Tedoire • Tedromeda • Theta
|slash||Deamus • Drarry • Flintwood • Grindeldore • Scorbus • Starbucks • Tomarry • Wolfstar|
|femslash||Bellamione • Parkwood • TNT|
|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|