I’D like to ask about my outdated green card. I have had the card since 1990 and used it for three years (paid taxes, etc). I then returned to Ireland and took up employment here. I have been back in the U.S. several times but not in the past five years or so.

My green card expired in 2000. I still have the card and am now wondering will that give me a problem were I to re-enter the U.S. for a holiday or business? Should I send it back to the authorities, and if so could you tell me how I go about this? I don't plan a journey to the U.S. just now, but I nevertheless wonder should the card be returned.
AS your green card is well and truly invalid at this point, you can, if you wish, contact the American Embassy in Dublin and file an I-407 form, which will formalize your abandonment of permanent residence in the U.S.
Actually, you have the option of filing the I-407 with a Customs and Border Patrol officer at your point of U.S. entry.   The form is only a page long, doesn’t cost anything to file and asks basic questions, including a reason why permanent residence was abandoned.
Given that your green card is so old at this point – and you also add that you’ve been back in the U.S. several times, though not during the past five years – you don’t have to file the I-407. If your past permanent residence here comes up, you can explain that Ireland is now your home and that your future trips to the U.S. will be short-term in duration.
You should also be prepared to provide evidence of this fact, if you’re asked. Items such as a mortgage statement or lease, bank records and an employment letter will suffice.
Those who held permanent resident status here in the past are not barred from returning to the U.S. as non-immigrants in the future. But they will be expected to meet all the usual standards for entry, including, as we said, proof that the trip is temporary only.
Ex-green card holders should also bear in mind that they remain eligible to apply for permanent residence in the future, if the opportunity to do so arises. Of course, they will be required to meet all eligibility requirements.