McCay represented Ireland at Under-16 and Under-18 levels before making her senior debut. She made her senior debut in January 2007 against Canada. In April 2011 she captained Ireland as she made her 100th senior appearance against France. In March 2015 McCay was a member of the Ireland team that won a 2014–15 Women's FIH Hockey World League Round 2 tournament hosted in Dublin, defeating Canada in the final after a penalty shoot-out. On 11 June 2015 McCay made her 200th senior Ireland appearance at the 2014–15 Women's FIH Hockey World League Semifinals in a game against Germany. In July 2015 she was also a member of the Ireland team that won the 2015 Women's EuroHockey Championship II. In the final against the Czech Republic, McCay made her 209th senior Ireland appearance. As a result, she became Ireland's most capped women's field hockey international, moving one ahead of Nikki Symmons. In January 2017 she was also a member of the Ireland team that won a 2016–17 Women's FIH Hockey World League Round 2 tournament in Kuala Lumpur, defeating Malaysia 3–0 in the final. In August 2017 she made her 250th appearance at the 2017 Women's EuroHockey Nations Championship.