Under FIFA rules, the Wigan midfielder could turn his back on Ireland's Euro 2012 campaign in favour of Scotland, the country of his birth, because he has only played in one friendly international and is still under 21.
McCarthy, 20, pulled out of the squad to play in the new Home Nations' tournament at the weekend, just hours after scoring twice in the 4-3 win over Blackburn.
Tough choice: McCarthy could yet play for Scotland or Northern Ireland
But sources close to McCarthy say he withdrew because of concerns over his fitness, as he continues his recovery from an ankle-ligament injury which kept him out for almost three months. He has played three games in the last fortnight but has yet to complete 90 minutes.
How it works The Carling Nations Cup was the brainchild of former Northern Ireland manager Lawrie Sanchez, who suggested in 2006 a way to revive theold Home International Championship without England by inviting the Republic of Ireland.
The Republic are hosts for the first tournament, with Dublin's Aviva Stadium the venue for all six matches in a league format that will wind up in the last week of May.
Ireland manager Trapattoni has admitted he has not spoken to the Glasgow-born playmaker, who is one of the young talents the Italian coach is planning to build his newlook squad around.
And his response to McCarthy's unexpected withdrawal added fuel to the conspiracy theories in Dublin that he is planning to abandon Ireland and pledge his allegiance to Scotland, who face Northern Ireland tomorrow.
Wigan boss Roberto Martinez has spoken to the Italian and assured him the late decision to pull out of the squad was necessary because McCarthy needs treatment at the club.
Fine servant: Given will win his 110th cap for Republic of Ireland against Wales
The teamsRepublic of Ireland (4-4-2): Given; O'Shea, Dunne, St Ledger, Clark; Coleman, Whelan, Gibson, Duff; Walters, Doyle.
Wales (4-3-3): Hennessey; Eardley, J Collins, D Collins, Ricketts; Vaughan, Crofts, King; Church, Earnshaw, Robson-Kanu.
Referee: M Courtney (N Ire).
TV: S4C from 7.15pm, Sky Sports 1 from 7.30.
Trapattoni said: 'I only spoke with Martinez and he explained, it is not the club and it is not the manager. He has an injury and he is young and I think at the moment he wishes to stay at home, with Wigan in a bad position in the league. I accept this and I understand.
'I do not want to be in conflict with the club. The federation could say you must come here under the UEFA rules, but it's better they come with enthusiasm and they are happy, it's better. We can also wait.'
But keeper Shay Given has, who will captain Ireland tonight for his 110th cap, said: 'When I was very young I was very excited to be in the squad for your country and maybe a young player's mentality has changed since I was 20.'
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Explore more:People: Shay Given, James McCarthy, Lawrie Sanchez, Roberto Martinez Places: Glasgow, Dublin, Scotland, Republic of Ireland, United Kingdom, Wales, Northern Ireland, Ireland