Born: circa 1980.
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.
Other names: Professor Binns calls him 'O'Flaherty' (CS9).
First year at Hogwarts: Gryffindor, 1991
House: The Sorting Hat took almost a whole minute to decide his house as Gryffindor (PS7).
Seamus is an 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." The Sorting Hat took almost a whole minute to decide his house as Gryffindor (PS7).
He's good friends with 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).
In the Movies
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).