He is one of twelve sons born and educated locally at Scoil Naomh Brid in Boardsmill and St. Michael's CBS in Trim.

Dempsey later attended University College Dublin where he was conferred with a Bachelor of Arts degree as well as a diploma in career guidance.

He subsequently completed a Higher Diploma in Education at St Patrick's College, Maynooth and worked as a career guidance teacher for many years.

Dempsey enhanced his local political profile at this time by also serving as a member of Trim Urban District Council.

In September 1991, Dempsey was a key member of the so-called "gang of four" which proposed a motion of no confidence in Charles Haughey as the leader of the party.

In 1992, Reynolds eventually became party leader and Taoiseach and Dempsey's loyalty was rewarded by being appointed Minister of State with responsibility as government Chief Whip.

During this period in opposition, he was heavily involved in key election strategies, most notably in the Dublin West by-election which was won by Brian Lenihan Jnr.

Following the 1997 general election Dempsey was a key player in helping to negotiate a programme for government between Fianna Fáil and the Progressive Democrats.

After agreeing to have an independent assessment of the raw gas pipeline planned for Kilcommon, Erris, Dempsey employed a firm to carry it out.

Only after the report was concluded did it emerge that the firm was half owned by Royal Dutch Shell, a fact which they neglected to relate to the Minister.

To that end his department also established a DTT trial in August 2006 to run for 2 years around Three Rock, Dublin transmitter and Clermont Carn, in County Louth.

Dempsey received widespread criticism across the Irish media over his January 2010 holiday in Malta and perceived lack of contactability when severe weather conditions led to massive disruption across Ireland's transport systems.