The LIRC lib/ directory is indeed code used in various places. It offers four views:

External C/C++ applications which neds to receive or send IR signals uses the lirc_client.h header file and links to liblirc.so using -llirc. This is a mature, reasonably stable interface. See Client API and client_manual.
External python code could from 0.10.0 use the new python API. See Python client bindings and client_manual.
User space drivers includes the lirc_driver.h header and links to liblirc_driver.so using -llirc_driver. The other way around lirc accesses the driver as defined in the driver.h file (struct driver) This interface is new and willl probably need some time to stabilize. See User-space driver API and Driver API manual excerpt.
Internal lirc applications includes the lirc_private.h header file and links to liblirc_private.so using -llirc_private. This interface is subject to change without notice.

For build and configuration there are two pkg-config .pc files: lirc.pc and lirc_client.pc. These provides the proper compilation and link flags. See drivers/default/Makefile for an example how to use.

The bundled excerpts from the manual are the same as in the LIRC manual, but also here to be readily available and linked by doxygen in the API context.