“I’m half-and-half,” said Seamus. “Me dad’s a Muggle. Mum didn’t tell him she was a witch ’til after they were married. Bit of a nasty shock for him.” -- Seamus Finnigan (PS7)
Seamus Finnigan is an Irish boy with sandy hair, who is in the same Gryffindor dormitory as Harry.
He is a half-blood: his father is a Muggle, and his mother didn’t tell his dad that she was a witch until after they were married, which was a “bit of a nasty shock for him.” The Sorting Hat took almost a whole minute to decide his house as Gryffindor (PS7).
His best friend is Dean Thomas. Seamus and Harry had quite a falling out on their first night back in fifth year (OP11), but eventually Seamus came around and joined Dumbledore’s Army (OP27). He also fought in the Battle of Hogwarts on May 2, 1998 (DH31ff). There was also tension between him and Harry in their sixth year, when Harry picked Dean over him as a substitute Chaser (HBP14).
The Finnigans (along with Dean Thomas) camped out in the Irish section of the campground at the 1994 Quidditch World Cup Final in a tent covered with live shamrocks (GF7). Seamus is a Kenmare Kestrals fan (OP11).
House: The Sorting Hat took almost a whole minute to decide his house as Gryffindor (PS7).
Ancestry: Irish boy with sandy hair, he is a Half-blood: his father is a Muggle, and his mother didn't tell his dad that she was a witch until after they were married, which was a "bit of a nasty shock for him."
In 2001 Rowling flashed a notebook during a BBC interview that showed her early notes on the students in Harry Potter's year and Seamus is noted as being a Pureblood. This information cannot be considered canon, however, because the notes conflict in too many places with the stories as they were actually published.
Seamus Finnigan has a Muggle father and a witch mother. He also has a cousin named Fergus, most likely on his mother's side as he is a wizard (HBP17).
Achieved O.W.Ls in Charms, Transfiguration, and Defense Against the Dark Arts, and moved onto the N.E.W.T level.
Other canon notes and references
Other names: Professor Binns calls him 'O'Flaherty' (CS9).
The role of Seamus Finnigan is played by Devon Murray (IMDb).
Movie running gag (not from books):
In PS/f, Seamus has an ongoing problem with his spells blowing up in his face. In one of the deleted scenes, he offered to undo the Leg-Locker Curse which had been cast on Neville, but was turned down. This offended Seamus, who announced that his eyebrows have completely grown back, then walked away to reveal that a patch of hair was missing on the back of his head. This running gag was entirely invented by scriptwriter Steve Kloves from a single reference in PS when Seamus ignited a feather in Charms class. Indeed, in the books it's Neville who melts cauldrons and has all the accidents with magic, not Seamus (PS/f).