CBR is a bit interesting.
The Internal Resort Shuttle is actually the Downtown Disney bus. The bus enters CBR as the IRS... and drops off DTD while picking up for IRS. Once it makes a full complete circle, it then drops off for IRS, and picks up for DTD.
The reason for this is CBR has 7 stops. Downtown Disney is unique in that it can have full bus loads coming AND going at the same time... (typical parks dont have that) By dropping off first, and THEN picking up... it makes sure there is room for everyone.
DTD/IRS run approx every 15mins... Typically... there's usually 2 DTD busses in CBR at any given moment... one doing the IRS, and one as DTD.