Are you going to the US on vacation, or are you going to be there for an extended stay?
For extended stay you need something like this:
www.vhi.ie
For a holiday, you probably need topup on the below for the US because that won't go very far in the US - unless your particular policy has a much higher max.
If you look at say VHI travel insurance, I think their cover assumes you already have health insurance in place (see
https://www.vhi.ie/travel-insurance) and is on top of that.
Your Vhi Healthcare health insurance plan covers all your eligible in-patient hospital charges up to a maximum* of €65,000 / €100,000 if you need emergency treatment while on holidays abroad.
*Check your policy documents for the maximum amount on your plan.
Secure Your Health Insurance Coverage Abroad with Vhi. Discover international coverage and easy claims processing for peace of mind while traveling.
www1.vhi.ie