The Republic of Ireland international struck deep into stoppage-time at Anfield, seconds after Divock Origi had a penalty appeal declined, as last season's runners-up were denied a return to the Wembley showpiece.
Just as they had in the first leg, Southampton spurned a host of chances to put the tie to bed, but it proved immaterial as their defence - missing the injured Virgil van Dijk - became the first side in League Cup history to reach the final without conceding a goal.
