Shannon signed for Shamrock Rovers U19s, from Home Farm, in mid-2011. A year on, he went out on loan to Dundalk, appearing for eight times for them in the league before returning to the Tallaght club at the end of that 2012 season.
The 2013 season saw Bohemians manager Aaron O’Callaghan bring Lorcan to Dalymount. He only made two appearances for the ‘Gypsies’ however, as Owen Heary replaced O’Callaghan as manager at Dalymount.
November 2013 saw Shannon join Longford Town and he played 24 times in the league for ‘deTown’ during the 2014 season, as they gained promotion to the Premier Division by winning the First Division. He re-signed for Longford for the following season and played in 21 league games for the club, as they went on to take the sixth position in the Premier Division.