Updated June 2011: Rory McCann has played bit parts in movies such as Solomon Kane, Season of the Witch and the recent remake of Clash of the Titans.
So far, they haven't featured The Hound a great deal, he's mostly stood in the background looking manacing. They have cut back scenes of him with Sansa but they did show the part when he fought his own brother "The Mountain". He's okay playing the role but I imagined the Hound having more of a gruff voice.
My fantasy cast for Sandor Clegane was Javier Bardem - that was way back when his most recognized film was No Country for Old Men. He's been in a lot of movies since then.
Sandor Clegane from A Song of Ice and Fire is the younger brother of Gregor Clegane and a retainer to House Lannister. He is nicknamed "The Hound," in part because his family arms feature three dogs, and in part for his savage nature and unquestioning obedience to his masters. He regularly wears a distinctive helm in the shape of a snarling dog's head. He is considered one of the most dangerous fighters in Westeros, though he frequently drinks himself into stupors. As a child, Sandor received gruesome facial burns when Gregor shoved his face into a brazier for playing with one of Gregors toys. He now has a phobia of fire, and loathes his brother, Gregor Clegane, and the hypocrisy of knighthood in general.
Sandor acted as bodyguard and servant of Joffrey Baratheon, who regularly called him "Dog". He won the Hand's Tourney after saving Loras Tyrell from Gregor's wrath. While drunk off his winnings, Sandor became enamored with Sansa Stark, but also mocked her for her polished manners, calling her a "little bird" who chirps on command. Sandor was eventually named to the Kingsguard, despite refusing to become a knight, and in this capacity he was often assigned to watch over Sansa. Despite his loyalty to Joffrey, he strategically defended her from Joffrey's wrath several times, and was the only member of the Kingsguard who was never commanded to beat her. Sandor also fought heroically during the Battle of the Blackwater, but eventually deserted after refusing to return to the flame-choked battlefield.
Javier Bardem came to mind while reading about Sandor Clegane. Even though he seems kinda sweet in interviews and is actually quite good looking, I've only seen Javier's acting in No Country for Old Man.