How does the time difference between US and UK affect the Irish-domiciled ETFs tracking the S&P500?

ETFs are bought and sold during market hours during which the market price of the ETF is determined by the value of the fund’s holdings as well as supply and demand in the market place for the ETF. While the share price is largely determined by the underlying value of the portfolio (known as the Net Asset Value or NAV), there may be some differences from time to time especially during times of market volatility.
The market price is different to an ETF’s NAV which shows the official value of the ETF once a day, based on the closing prices of the underlying securities. The NAV is used to measure ETF performance.
An ETF is said to trade at a premium when its price exceeds its NAV. An ETF is said to trade at a discount when its price is below its NAV. Premiums and discounts are usually negligible for the majority of ETFs but they can be large during volatile times.