When an existing member resigns or dies, a member is co-opted to replace them. By convention, someone from the same party is co-opted, although the controlling group on the council may appoint whomever they wish. There is no provision for local authority by-elections. Ministers of State have usually relinquished their council seats on appointment.
Following the 1999 local elections, legislation was introduced to abolish the 'dual mandate' where TDs and Senators also served as local councillors (Local Government (No2) Act 2003). Anyone holding a dual mandate qualified for a one-off payment if they resigned their council seat.