Some machines have more than one IP address with direct access to the internet and/or the internal network. By using the -b, -i and -o options, you may choose which of them are used and when.
The -b option tells the proxy on which IP address to listen for incoming IRC connections. By using this option you can make sure that Tircproxy is only listening to your internal address, which could suffice to keep outsiders from attempting to use the proxy. This flag is incompatible with inetd operation.
The -i option tells the proxy which IP address to use for DCC communication with it's clients. This value can be autodetected, except on Linux in transparent mode (because of how transparancy is supported by the Linux kernel).
The -o option tells the proxy which IP address to use to connect to a remote server. Thus if the host running Tircproxy has two external addresses, e.g. 1.2.3.4 (example.com) and 1.2.3.5 (example.org), you can choose which is used for communication with the Internet (both IRC servers and external DCC). This lets you choose whether the proxy users appear to come from example.com or example.org. Without this option Tircproxy will let the operating system decide which address to use.
Using the above addresses and the added assumption that the machine's internal addresses are 10.11.12.13 and 10.11.13.14, these flags could all be tested at once by running Tircproxy like this:
tircproxy -d9 -s 7666 -i 10.11.12.14 -b 10.11.12.13
-o example.com -MILRH irc.undernet.org 6667
Trying to connect to port 7666 on most of the machines addresses should fail, only connecting to 10.11.12.13:7666 will open a connection to IRC. That connection would originating from example.com. The interface 10.11.12.14 would be used for internal DCC-related communication.