While in college Diarmuid played an active role in the Student's Union. In 1990 he was elected to Class Reps Council. In 1991 he became the Equal Rights Officer and in 1992, the students of the University of Limerick elected him as their President.

The outstanding achievement of Diarmuid's year in office was the introduction of formal repeat exams for the first time in the university's history, but there were others. He inherited a union with a £25,000 debt and left it with a significant surplus. He served as the first chairman of ULSU Services Ltd, and oversaw the opening of the Student's Union shop and print shop. He reformed the workings of Class council and the funding of clubs and societies. As a result of his successes in Limerick, Diarmuid was later hired by the students of Waterford Institute of Technology to assist in the re-structuring of their Students'Union.

Diarmuid represented Ireland with distinction at three World debating championships, Trinity College in 1991, Oxford, 1992 and Melbourne 1993. At the Oxford competition Diarmuid and his team mate, Niamh Lyons were ranked joint 17th out of the 500 best public speakers in the world.

Prior to devoting himself to politics fulltime, Diarmuid spent five years with Sykes in Shannon, where he worked as a technical support supervisor on the EircomNet, Motorola and Gateway accounts. He managed teams ranging in size from 9 to 45 people .

In 1999, Diarmuid became the first person from the Singland area ever to be elected to Limerick City Council. He has worked hard to ensure that the long neglected problems of the area finally receive the attention they deserve.

In August of 2000, Diarmuid married his fiancée, Ruth Keevey of Bruff, Co. Limerick. The couple now live at 19 Lynwood Park with their first child, Michael.

Diarmuid's father Tom Scully is originally from O'Donoghue Avenue in Janesboro and his mother, Mary Cleary is from Ballynanty Beg, Ballynanty. Diarmuid has three brothers - Brian, Ciaran and Tomas and one sister, Olivia. The family has been heavily involved in athletics in Limerick. Diarmuid's grandfather, Paddy Cleary was a national champion at several events, while his granduncle, Tommy Cleary was one of the founders on the Limerick Athletic Club. Ciaran was Irish junior Heptathlon Champion for three years running, while Olivia won the gold medal for Ireland in the high jump at the AAA games in Crystal Palace and at the TASA games in Thailand. Brian has served as chairman of the Limerick Chess club and is a committee member of the Shannon rowing club. Tomas is currently studying Philosophy in NUI, Galway.