It is the first time the beer brand has teamed up with a food company to launch a product for mass retail. The dark-coloured bread has 17% Guinness content and a malty taste.
"The challenge was to create a bread whose appearance, texture and taste fully reflect and complement the distinctive integrity and taste of the beer," said Michael Murphy, Irwin's business development controller.
David Berry, licensing director for Diageo Europe, said: "We have been keen for some time to harness the qualities and taste of Guinness into suitable food products and bread was a natural starting point.
"We hope it will pave the way for further mass-market food product development for us."
Guinness bread (rsp: £1.50 for a 400g loaf) is listed in major multiples.