A strong black cup of tea sweetened with milk and a spoonful of sugar is a drink that warms the Irish heart. Irish breakfast tea is made from leaves grown in Assam, India. Ideally, these tea leaves should be steeped at 212 degrees Fahrenheit for the characteristically strong brew. The caffeine content in Irish breakfast tea is perfectly within the limits of moderate caffeine consumption, or 300 milligrams per day.
A 6-ounce cup of black tea, such as Irish breakfast tea, typically contains around 40 milligrams of caffeine. For comparison, the same serving of regular coffee has between 79.4 and 130.7 milligrams of caffeine -- double to triple the amount in tea.
It's How You Brew
Adagio Teas explains that a number of factors influence the caffeine content in a cup of tea. Dry green, white and black tea leaves have a similar caffeine content, but black teas, including Irish breakfast, are brewed at much higher temperatures, which means higher caffeination. The longer you steep Irish breakfast, the more caffeine your cup will have.