The Chief of Staff Lt Gen Dermot Earley is retiring early due to ill health and is to be replaced by Major General Sean McCann.
He is being succeeded by one of his current deputies, Major General Sean McCann.
General Earley, 62, was not due to retire until next February. However, he has been suffering from a debilitating illness since last autumn.
He had signalled his intention to stand down seven weeks ago after he was awarded the Distinguished Service Medal by the Taoiseach.
Dermot Earley played Gaelic football with his native Roscommon for 20 years from 1965 to 1985 and won two All-Star awards.
He later went to manage Roscommon as well as Kildare, where he now lives.
In a distinguished military career, he was also one of the first members of the army ranger wing. He served for four years as a senior military adviser at UN Headquarters in New York, and twice in Lebanon.
The new Chief of Staff, General McCann, is a native of Co Cork but was raised in Co Tipperary.
A cavalry officer, he joined the army in 1970 and studied at University College Galway and Fort Leavenworth in the US.
General McCann has served overseas on six occasions, including Lebanon, Israel, Iraq/Kuwait border and the former Yugoslavia.
He played rugby for UCG and Connacht and captained the Defence Forces and the Curragh (now Newbridge Rugby Club).
Both General Earley and General McCann live in Newbridge, Co Kildare.
Defence Minister Tony Killeen said he looks forward to working closely with General McCann and he thanked Dermot Earley for, what he said was, his truly remarkable contribution to public life.