Hi Pat,
You won't need a visa if you are sitting in Ireland doing the work.
If you will travel to the US location / office ocassionally you will need to get an ESTA, which is a piece of cake (unless you are a some class of deviant) & costs USD21.
Tax situation: You will be liable to pay income tax in Ireland. You will not be liable to income tax in USA.
Find out if the role can actually be serviced from Ireland.
East coast USA will start their workday at around 12.00 Irish time & will wrap it up around 20.00-21.00 Irish time. Are you good with that if they require you to be available during their work hours? Synchronous hours.
West coast USA is obviously a later start & a later finish.
They may not need you to keep the same hours as them - find out. Asynchronous hours.
If you are the 1st employee in Ireland there may be an overhead for them to set up payroll for you & also any benefits may need to be set up. This is a cost to the company if they even provide it for contractors.
good luck!