[2] O'Sullivan joined the Department of Foreign Affairs in 1993 after working for some years in Hong Kong.
[3][4] Before becoming ambassador to Israel served as Consular Director at the Department of Foreign Affairs and Trade in Dublin.
Before that he held two ambassadorial postings in Nigeria and Indonesia and a period as Director for EU Policy in the office of the Taoiseach (Prime Minister).
[8] In 2019 O'Sullivan became Ireland's ambassador to Israel, replacing Alison Kelly, who retired from government service.
[9] He presented his credentials to Israel's president, Reuven Rivlin, on Wednesday, August 7, 2019.