A state of emergency was declared in Ireland on 02 September 1939 following the outbreak of the Second World War. The Emergency Powers Act 1939 was passed by the Oireachtas the following day. The Emergency (Ré na Práinne) necessitated a large scale mobilisation of the Defence Forces to face a potential invasion of Ireland by either of the belligerents. Contingency Plans were developed to resist an invasion by British Forces from Northern Ireland and  to resist an invasion by German Forces on the south coast. The Blackwater Manoeuvres were held to prepare for the latter from the 17^th of  August to the 27^th of September 1942 .

Twenty thousand members of the Defence Forces took part in the manoeuvres with the First (Thunderbolt) Division under Major General M.J. Costello, a founding member of ONE, defending the line of the Blackwater River and the Second (Spearhead) Division under Major General Hugo McNeill deploying on foot from the Dublin / Kildare / Westmeath area as the offensive force.

The First Division with its headquarters in Collins Barracks Cork had three Brigades:

• 1^st Brigade: HQ Clonmel – 10^th, 13^th and 21^st
• 3^rd Brigade: HQ Cork – 4^th, 19^th and 31^st
• 7^th Brigade: HQ Limerick – 9^th, 12^th and 15^th

The Second Division with its headquarters in Carton House Maynooth had three Brigades:

• 2^nd Brigade: HQ Dublin – 2^nd, 5^th and 11^th
• 4^th Brigade: HQ Mullingar – 6^th, 8^th and 20^th
• 6^th Brigade: HQ Dublin – 7^th, 18^th and 22^nd

In addition there were two other independent brigades and three garrison battalions.[1]

The RTE Archive clip below provides a flavour of the exercise.

https://www.rte.ie/archives/2017/0808/895947-irish-army-blackwater-manoeuvres/

[1] A History of the Irish Army – John P. Duggan – Pages 198 to 209