ITV confirmed that Richard - who is also currently starring in hit US drama 'Girls' - will play Simon Bricker who visits Downton as a guest of the Granthams.
Anna Chancellor will also have a guest role, playing Lady Anstruther. The BAFTA-nominated actress's latest role comes hot on the heels of her recent success in London's West End in 'Private Lives'. She is more widely known for her roles in 'The Hour' and 'Four Weddings And A Funeral'.
Another new addition to the cast is internationally acclaimed actor Rade Sherbedgia ('Eyes Wide Shut', 'Snatch', 'The Saint' and 'Taken 2') who will play a refugee who has fled the Russian revolution after the First World War.
Dame Harriet Walter will also be making a return to reprise her role as Lady Shackleton, along with Peter Egan, returning as Lord Flintshire.
'Downton Abbey' Executive Producer Gareth Neame, said: “We are delighted to welcome these talented actors to the world of Downton. The characters they play are set to bring yet more excitement and intrigue to the show."
As well as filming an eight-part fifth series, Downton Abbey will return at Christmas 2014 with another feature length episode.