I've found four marriages in the
BMD Index for a John Gibbons in the right registration district for Tallaght (i.e. Dublin south) around the right time.. but I cant find any match for any of these to the bride's surname of Gorman.
Name: John Gibbins
Registration District: Dublin South
Event Type: Marriage
Year: 1863
volume : 2 / page : 860
Name: John George Gibbon
Registration District: Dublin South
Event Type: Marriage
Year: 1863
volume : 5 / page : 354
Name: John Gibbons
Registration District: Dublin South
Event Type: Marriage
Year: 1866
volume : 17 / page : 677
Name: John Gibbons
Registration District: Dublin South
Event Type: Marriage
Year: 1871
volume : 2 / page : 834
If they were Catholic and married before 1864 they would have missed Civil Registration, so you'd need to try parish records.
Shane