This is a very complex area and you'd need to find a solicitor familar with 'conflict of laws'. Probably you need an Irish will to bequeath your Irish property and ditto for the US property. Then there are Irish rules that come into play, not sure but I guess they'd be based on your legal status in Ireland, so domicile might come into play. For example you cannot disinherit your spouse, but you can your children, etc.
A first port of call might be the US embassy. Or ask the Law Society for recommendations of an Irish solicitor who is expert on this area.