Head coach Andy Farrell has retained a 35-man group for the USA’s visit to Dublin.
Munster centre Farrell requires assessment on a head injury after being forced off during Saturday’s 39-31 win over Japan, while Leinster back Larmour suffered a groin problem in that match.
The pair will return to their respective provinces for treatment.
Experienced Munster back row O’Mahony has been permitted to leave the camp and begin his off season.
Head coach Farrell has retained a 35-man group for the USA’s visit to the Aviva Stadium in Dublin.
Uncapped contingent Robert Baloucoune, Caolin Blade, Harry Byrne, Tom Daly, James Hume, Michael Lowry, Paul Boyle, Peter Dooley, Ross Molony, Tom O’Toole, Nick Timoney and Fineen Wycherley will bid to make international debuts in their country’s final fixture of the summer.
Munster forward Gavin Coombes was the sole Irish debutant in the entertaining five-try success over Japan, which was secured by scores from the now-departed Farrell, Stuart McCloskey, Finlay Bealham, Josh Van Der Flier and Jacob Stockdale, plus 14 points from the boot of Joey Carbery.
The upcoming clash with the USA is the third meeting between the two nations in the past four years, with Ireland winning 55-19 in New Jersey in 2017 before triumphing 57-14 in Dublin a year later.
Gary Gold’s visitors travel across the Irish Sea following Sunday’s 43-29 Twickenham loss to a much-changed England side.
Ireland and the USA are also due to meet in Las Vegas in late October.
Ireland squad: W Addison (Ulster), R Baloucoune (Ulster), C Blade (Connacht), B Burns (Ulster), H Byrne (Leinster), J Carbery (Munster), C Casey (Munster), A Conway (Munster), S Daly (Munster), T Daly (Connacht), J Gibson-Park (Leinster), J Hume (Ulster), H Keenan (Leinster), M Lowry (Ulster), S McCloskey (Ulster), J Stockdale (Ulster); R Baird (Leinster), F Bealham (Connacht), P Boyle (Connacht), E Byrne (Leinster), G Coombes (Munster), U Dillane (Connacht), P Dooley (Leinster), C Doris (Leinster), D Heffernan (Connacht), R Herring (Ulster), R Kelleher (Leinster), D Kilcoyne (Munster), R Molony (Leinster), T O’Toole (Ulster), James Ryan (Leinster, capt), John Ryan (Munster), N Timoney (Ulster), J Van Der Flier (Leinster), F Wycherley (Munster).