Taxes & Work Authorizations for US Citizen Working in US for Ireland-based Company

I can answer the last part of your question—as a U.S. citizen, you have to report and pay tax on your worldwide income. However, you get a credit on your U.S. taxes for foreign taxes paid, if it turns out that Ireland does tax this income, so you won't be double taxed on it. (Your net tax will be the higher of the Irish tax rate or the U.S. tax rate.)

I live in and would work from the US

THIS is all that matters.

You are in the US, you work from the US, so US work laws apply. Period. YOu do not need a visa for ireleand because you work in the USA. The government of Ireland sort of does not care, allthe irish company must do is register an office in the USA to employ someone and follow the same regulations as any US company. THere are no irish taxes for you - the company deducts all payments to you as expense and has to follow US regulations. Generally a freaking pain in the ass - for one person. An alternative is that the company hires you through an umbrella company that does all above and writes an invoices to the irish company once per month - waaaaaaaay easier for one employee. Seriously, the idea of hiring one foreign country based person gives me creeps - the amount of paperwork is insane for ONE person. I would never do that and/or always go through an umbrella which handles all the paperwork for me.

Obviously all income falls under US income tax because - regardless of the US concept of taxing all your worldwide income - you actually live AND WORK in the USA anyway, so this case is trivial. You essentially are an US employee.

Unless you work as "contractor", then you are responsible for a lot and write an invoice to the company every month. Again, no irish laws applying. Sole propietor, LLC, whatever you decide. YOu write an invoice then and handle all local taxation and write off your equipment etc.