As long as your mother was US resident at the time of her death, you need to ignore the usual Irish CAT rules and look to the Irish US Double Tax treaty under which you would have an exemption of up to $5,000,000 US from Federal Estate tax and that should discharge your liability to Estate Taxes in both jurisdictions.
However, as a US Citizen, you are caught by the FACTA reporting requirements designed to ensure that you appropriately file your US Taxes with the IRS each year (something that many US Citizens in Ireland "forget" to do) you must file with the IRS annually even though you have been in Ireland for the last 18 years and may have no earned income.
You therefore have some complexity around your personal financial affairs on an ongoing basis even if all you do is use the inheritance to pay off debts.