Are Irish domiciled S&P500 ETF “less liquid” and have higher tracking error than US domiciled S&P ETFs?

This depends on the individual ETF and on the stock exchange.

With f.ex.

ISIN IE00B3XXRP09 (VUSA, VUSD) there should be no liquidity/spread problem on XETRA or London Stock exchange.

For currency conversions (to EUR, USD) for buying it, don't let that do your bank or broker, but a service as f.ex. TransferWise or maybe CurrencyFair.

The tracking error with these 'giant' ETFs is a theoretical construct. For practical purposes it should be no real problem.