The experienced midfielder joined Town in 2016 after two years with Southend United.
Originally beginning his career with his hometown club Shelbourne, Deegan enjoyed spells with Longford Town, Galway United and Bohemians before moving to England to join Coventry City in 2010.
In two and a half seasons with the Sky Blues, he made 42 Championship appearances, scoring 5 times.
Deegan moved to Hibernian in the summer of 2012, but only stayed in the Scottish capital for one season before joining Northampton Town.
He moved onto Southend United in 2014, helping them gain promotion to League 1 alongside Town.
Most commonly deployed as a defensive midfielder, Deegan went onto make 25 appearances for The Shrimpers last season and it is hoped he will add some bite to the Town midfield.