On balance, you'd have to say that it would be Fiona Cleary. Whatever you might think of her sordid past, there's no doubt that she's displayed some heroic qualities in making the sacrifice to marry Paddy Cleary and give her illegitimate son, Frank, a name. At that time, being born out of wedlock was considered a scandal as well as a sin, so Fiona did right by Frank in not exposing him to the stigma of illegitimacy. In doing so, she partly atoned for her adulterous liaison with a married man.
As for Luke O'Neill, it's safe to say that the word "heroic" doesn't exactly leap to mind when one thinks of him. Heroism implies sacrifice, putting yourself before others as Fiona did with her illegitimate son Frank. But that's not like Luke at all. He's a money-grubbing mercenary who's always looking out for number one.
After he marries Meggie, he puts his own selfish needs ahead of hers every time. As well as being a notorious skinflint, Luke shows his contempt for Meggie by hiring her out as a maid—this, despite the fact that Meggie brought a substantial some of money into the marriage (which, of course, Luke has appropriated for himself).