Irish breakfast tea is a blend of several black teas, most often a combination of Assam teas and Ceylon teas. Irish tea brands, notably Barry's, Bewley's, Lyons and Robert Roberts in the Republic and Nambarrie's and Thompson's Punjana in Northern Ireland are heavily weighted towards Assam. It is one of the most popular blended teas, common in tea culture in Ireland. When tea was first transported from China to Ireland in the mid-18th century, it was mainly introduced to the wealthy as a result of its high cost and low demand. However, throughout the mid-19th century, Irish breakfast tea became readily available to those of both lower and higher socioeconomic classes.

Assam tea is a black tea named after Assam, India, the region of its production. It is manufactured specifically from the plant Camellia sinensis var. assamica (Masters). The Assam tea plant is indigenous to Assam—initial efforts to plant the Chinese varieties in Assam soil did not succeed. Assam tea is now mostly grown at or near sea level and is known for its body, briskness, malty flavour, and strong, bright colour. Assam teas, or blends containing Assam tea, are often sold as "breakfast" teas. For instance, Irish breakfast tea, a maltier and stronger breakfast tea, consists of small-sized Assam tea leaves.