Friendlies only cap tie if you are only eligible at the time of a friendly to play for one NT. If you are eligible to play for more than one nation at the time of the friendly, it does not cap tie you. For example, last year when Poku got cap tied when he got called up for Ghana last season, because Ghana was the only NT he was eligible for. So even if he gets his US Citizenship later on, he would not be eligible for USMNT. If he had US citizenship at the time of the friendly, he would not have been cap tied.
My guess is if T-Mac has an Irish passport right now, playing in a friendly would not cap tie him to US or Ireland until he plays in an actual competition.