IRLP, the Internet Radio Linking Project, was developed by Canadian Ham Dave Cameron, VE7LTD because existing Windows based amateur linking software used VOX and had no security from non-amateur access. The IRLP network uses digital switching to create an instant and secure link between licensed repeaters around the world.
IRLP sends receiver audio to a sound card - which converts it into ADPCM digital data. The Linux PC then converts this data into digital packets with IP addresses for the destination node and sends them over the internet to a destination Linux PC decodes and sends them to the sound card and out to the transmitter microphone of the link radio which then transmits the audio out over the local repeater. The transmitter is keyed as soon as these TCP/IP (Internet Protocol) packets start to arrive. When the data stops the link radio reverses the process automatically.
PGP keys are assigned to each node to establish security. When a connect request is made the connecting node exchange key challenges which must be correct or the connection is refused.
IRLP, unlike EchoLink, does require the use of a radio.
Much more information here: