Brought up in Ballynoe in County Cork, Casey's father was a council worker and farmer, while her mother worked as a public health nurse at a time when it was highly unusual for both parents to work outside the home. She has one sibling; a younger sister, Terry. Excelling at school, she went on to study medicine at University College Cork and soon after graduation met her husband, John McGuiggan.
Casey has written five books on psychiatry and is editor of the British Psychiatric Bulletin. Her clinical and research interests include depression and suicide prevention. From 1994 to 1999, she chaired the Irish Fitness to Practise Medical Council Committee. In 2007, The Dubliner magazine listed her in their "Good Doctor Guide".
Casey is a practising Roman Catholic. She sits on the board of the Iona Institute; a conservative Catholic think-tank which she helped to found. Casey is known for her opposition to divorce, advising the Irish government against holding a referendum to legalise divorce in 1995. She also maintains that "the sense of loss children feel when parents separate is greater than when a parent dies". She does not, however, disagree with divorce in the case of a violent or abusive spouse. Casey also opposes abortion, surrogate pregnancy, anonymous donor in vitro fertilisation,, non-traditional family units, and same-sex marriage, as well as being a proponent of adoption. She has testified in front of the Irish Government, at the British House of Commons and in Irish legal cases on a number of these issues. Casey also writes a regular opinion column for the Irish Independent newspaper and in the past has contributed to the Sunday Business Post and to the letters page of the Irish Times, as well as appearing on national television and radio.
