McDonnell first appeared for the Republic of Ireland in three friendly games against the United States in September 2008. She said, "After college I played in Boston for a summer and it turned out my coach was Irish. He literally walked up to us on the practice field and asked if any of us were Irish, and one thing led to another. It’s been great." Mary Therese and elder sister Shannon featured in Ireland's qualifying campaigns for Euro 2009 and World Cup 2011. Daughters of Ian and Jean McDonnell, their eligibility for the Irish team derived from their grandfather; who was from Foxford.