Denis graduated from University College Dublin with a degree in Law in 1977. He served his apprenticeship with a firm of solicitors in Drogheda, County Louth. He was admitted to the Roll of Solicitors in 1978. He set up the firm of Larkin Tynan & Co. with Edward Tynan in 1982. He practised as a Partner in the firm from 1982 to 2016, when he retired as a Partner. He continues to practice as a Consultant Solicitor in the firm. Denis has extensive experience of general practice, specialising in commercial and residential conveyancing, wills and probate.