McCarthy goaled inside the first minute and the Dockers never trailed as they charged to a 7.13 (55) to 4.1 (25) victory at Sydney's Henson Park.
Five of Freo's seven goals were kicked by McCarthy (three) and Lally (two), all of them well executed, though their teammates were more wasteful.
The result gave the Dockers a 6-0 away record for the campaign, a stark contrast to their 1-3 log at home, where they can clinch a finals position next week with a win over the Western Bulldogs.
McCarthy adds a third goal to go with her 20 disposals 🌟— AFL Women's (@aflwomens) #AFLWGiantsFreo pic.twitter.com/JaNMMNBb2HOctober 26, 2024
GWS, playing their final home fixture of the season, suffered a seventh straight loss and stretched their winless streak to nine.
McCarthy also topped her team's disposal count with 22 and totted up a game-high 456 metres.
Lally's 390m gained was the second best in the match.
They are among a five-strong Irish contingent at Freo, with McCarthy revealing all of their families were currently in Australia and had flown from Perth to Sydney for Saturday's game.
"It's brilliant, hopefully now with this win and maybe next week we'll solidify the finals and they can stay on and support us through that as well," McCarthy told Fox Footy.
Defender Emma O'Driscoll was another top performer for the Dockers, pulling off several intercept marks and gaining 346m.
Freo opened up a 15-point lead at the first change.
Fremantle's Gabby Biedenweg-Webster shows her Giants opponents a clear pair of heels. (Dan Himbrechts/AAP PHOTOS)
Two second quarter goals from Aliesha Newman slashed the Giants' deficit to five, but the visitors responded by kicking the last two goals of the term to lead by 19 at the half.
Freo extended their advantage to 25 by three-quarter time, with the only major of the term coming through Ariana Hetherington - her first AFLW goal.
GWS increased their pressure in the second half but couldn't convert that into scoreboard rewards.
Giants stalwarts Alyce Parker and Rebecca Beeson never stopped trying.
Parker amassed a game-high 32 touches and added eight clearances and seven tackles, while Beeson racked up 19 disposals and nine tackles.
Acting Dockers captain Ash Brazill was taken out of the game in the third quarter due to ankle soreness.
