My previous post talking about the casting clue released on George R.R. Martins' "Not A Blog" has confirmed that Roger Ashton-Griffiths will be playing Lord Mace Tyrell.
Mace Tyrell is Lord of Highgarden, and Warden of the South. He is the son of the amazing Olenna Tyrell, The Queen of Thorns, who is played by Diana Rigg in Season 3. He is also father of Ser Loras, and Margaery. He's also father of Willas and Garlan Tyrell, but I'm not positive if we'll ever get to meet them in the show, I do hope so though.
I think Roger Ashton-Griffiths will do an outstanding job as Mace Tyrell, as he is an extremely experienced actor. And he has a great cast to work with while he's in Kings Landing; Peter Dinklage, Diana Rigg, Charles Dance and so many more. I think he looks pretty close to how I've imagined Mace Tyrell. The only faults I have is I imagined Mace to be a bit larger, and Roger is about 12 years older than Mace is. But to be honest with the entirety of the cast I have no real complaints.