The RC baptismal records for Drumcliff (Ennis) parish start in March 1841 https://registers.nli.ie/parishes/0782 You did not provide approximate ages for John and Margaret but I assume they were around 20 when they left Ireland in 1877? There is a baptismal record for a John McMahon June 10 1856 at Ennis with father Michael McMahon and mother Bridget O'Connor. I searched the subscription site Roots Ireland and this John had older siblings Joseph 1851 Catherine 1853 and younger siblings Thomas 1858 Ellen 1861 but no Margaret and no other brothers.
I then did a search for Margaret in Ennis with father Michael and the only record was in 1866 which seems too late for your interests.
I can expand the search outside of Ennis but can you let us know how old John and Margaret were when they left. Thanks!