Hi
Payments made by the individual into an IRA, are made after tax relief is given to the individual by the employer.
Payments from this pension are taxable in the USA.
HMRC do not recognise IRA schemes as pensions, so for UK residents, they are taxed as income under the interest and declared as foreign interest on a tax return (SA106).
Please also refer to the double taxation treaty with USA -
USA: tax treaties