Which broker should I choose to manually invest about 5k every quarter in Ireland domiciled S&P500?

Perhaps you can look into Saxo and IBKR and see which you would prefer.

Depends on the stock exchange universe the individual brokers offer, on the trading & account maintenance fees and importantly whether the broker accepts customers from your residency country (probably Singapore).

Probably Saxo and Standard Chartered could give you direct exposure to f ex German Stock Exchange (XETRA)