Grehan is a surname. Notable people with the surname include:
Ceara Grehan, Northern Irish theatrical and opera singer Danny Grehan (born 1965), Welsh actor Derry Grehan (born 1957), Canadian guitarist Francie Grehan, Gaelic footballer James Grehan, Australian singer and songwriter Martin Grehan (born 1984), Scottish footballer Stephen Grehan (born 1971), Irish retired hurler Stuart Grehan (born 1992), Irish golfer