With over 100 caps for the Republic of Ireland national team, O'Sullivan would bring a wealth of experience to United’s midfield. During her tenure at North Carolina Courage, she has secured two NWSL Championships, two Challenge Cups and three NWSL Shields.
She has previous experience in the Women's Super League, having had a loan spell with Brighton in 2020. This move was facilitated to ensure her availability for Ireland’s rescheduled Euro 2022 qualifiers, as extensive travel between the U.S. and Europe would have subjected her to prolonged quarantine periods.