I like George Clooney, but theres no way that he should play Sam Fisher. He's more of a comedic guy and Sam is a serious role. I don't think I could take him seriously.
Ironside obviously has the voice but he can't carry a film on his own. Plus he's much better at playing bad guys in movies.
Casting Sam Fisher isn't easy. There aren't many middle aged actors that fit the bill. The person that I immediately think of is John Travolta but I can't stand him plus he's put on a lot of weight. Michael Madsen might be okay but he's got the same problems as Ironside. Then of course you could go with people who have done similar films in the past like Val Kilmer (The Saint), and Tom Cruise (Mission Impossible). If worst comes to worst they could choose someone younger and do a prequel. If thats the case they should go with Christian Bale or Matt Damon.