Sep Lambert made his debut for Ireland against I Zingari in August 1896. Never a regular in the Irish side, his international career contains several large gaps between matches, and it was three years before he played his second match for Ireland, also against I Zingari in August 1899. Another three-year gap followed, before he made his first-class debut for Ireland, against London County in May 1902, in what was also Ireland's first first-class match. This was a rare period of consistent selection for Ireland, and he played three further first-class matches again that month, against the MCC, Oxford University and Cambridge University.