Hi PP4. I'm pretty sure they do, but you might well have to call them to confirm and book as it is probably not a very common choice for their online booking engine.
Yes I am sure they can, I seem to remember phoning them for a quote when I was just looking for a 10 night holiday and a cruise and they were able to help, but as Simon says you have to do this by phone.
I went on a 3 week holiday with them, they do not actually do holidays for 3 weeks because of the flights so they booked us on a british airways flight but everything else (villa, car etc) was through tcd.
Learned my lesson now though book everything independantly myself.
Last year when we went on a group holiday the flights were booked with TCD for 2 weeks but later we all decided we would prefer 3 weeks, when we tried to do this we couldnt has TCD said they work in 2 week blocks.
They do book 21 night holidays, but if you are using TCD as they have lower prices when you get to the 21 day mark then the difference is less noticiable and may be cheaper elsewhere.