But I have read that these people just abstained from the election, not that they voted Republican.
Yeah but the reduced turnout obviously drove up the proportion of those voting Republican. It would be like a Safe Republican in say Oklahoma winning like 40% of the black vote against an utterly incompetent Democrat in a low turnout year.
Obviously, voting Republican was still a mortal sin to many in the ethnic Irish (emphasis here to distinguish from the mainstream protestants who were pretty much WASPs by this point) community, but sitting on their asses was pretty much all the endorsement in the world that Harding needed.