McMahon, who had already won five national titles on Friday, added three more to her account on Saturday to win Swimmer of the Meet and cement her position as the future of Irish swimming.
The London Olympian set a new Irish record in the 200m breaststroke final to dominate Irish breaststroke rankings, now holding records in 50m, 100m and 200m.
Her time of 2:25.23 knocked Fiona Doyle's record, set earlier this year, from the books.
McMahon won her first gold of the day in the 400m IM final hitting the wall to finish in 4:47.80 - 12 seconds ahead of her closest competitor Lydia Kehoe (5:00.30) of New Ross, who took silver.
She then went on to battle it out with Bethany Carson in the 200m Freestyle final.
Carson and McMahon were neck and neck right to the end with Sycerika clinching gold in 1:59.63.
Lisburn's Carson took the silver medal just .45 of a second behind McMahon in 2:00.08.
Tallaght and NAC High Performance Centre swimmer Brendan Hyland continued his recent charge of breaking Irish junior records when he set a new standard for the 400m individual medley in the morning heats session, finishing in 4:22.86.
The 18-year-old then went on to take over two seconds off that time in the final when he cruised home over 10 seconds ahead of the competition, touching in 4:20.43.
Another in-form junior, Brendan Gibbons, set yet another Irish junior record in the 800m freestyle final, battling it out with Aer Lingus' Conor Turner (8:12.73), Gibbons held on for gold in 8:11.00.
UCD's Shauna O'Brien set a new Irish senior record in the women's 50m butterfly final.
O'Brien broke Julie Douglas' eight year old record of 27.40 when she touched in 27.30 to claim gold ahead of Kilkenny's Jane Roberts (28.27).
O'Brien's UCD team-mate Aisling Cooney (1:01.62) won gold in the women's 100m backstroke final, with NAC's Brian O'Sullivan (55.84) victorious in the men's equivalent.
Bangor's women (4:23.40) and Aer Lingus' men (3:55.00) set new Irish junior club records in the 4x100m medley relay final.
Bangor (4:23.40) then went on to set another junior club record in the women's 4x100m freestyle relay when they finished second behind Galway (3:58.32).