1897 Dec 18 Born Achill Mayo. I struggle to see when he was born. Ancestry trees offer a number of daes from 1891 to 1898. The RIC pension register gives 1897, but oddly the RIC Service Register has no date at all. His Constabulary medal was auctioned in March 2020 for £3300 and the auction gave 18 Dec 1897 as date of birth
The only date on GRO that makes sense is below at 10 Dec 1891. Which is a long way from 1897 in RIC. Or the 1897/1898 implied by his death. It is almost if he has taken a new identity

1901 census at Shraheens, Dooega, Mayo,

He enlisted in the British Army during the First World War.
1920 Jun 29 enlisted in RIC as # 71754. Clerk & ex-Soldier
1920 Aug 1 posted Limerick
posted Kerry
1921 Feb 26 The incident for which Gallagher was awarded the Constabulary Medal was reported in the local newspaper “The Liberator” of 1 March 1921: ‘About 7.30 p.m. last Saturday evening [26 February], a party of seven constables with Head Constable Maguire and District Inspector Captain Hamilton were ambushed at Coumcaoch on the Connor Hill road between Cloghand and Dingle. At this point, which is about 4 miles from Dingle, perpendicular cliffs hundreds of feet in height overhang the road, while from the other side a valley hundreds of feet in depth, abruptly descends. On the night in question the police, who were cycling at the time on their way to Dingle, were met by a hail of bullets from the cliffs overhead. The police at once dismounted, abandoned their bicycles on the roadside, and took cover behind the rocks. It was quite dark, especially in the shadow of the mountain, and to this fact the police owe their miraculous escape. Head Constable Maguire was somewhat severely wounded in the stomach, and Constables Stalford and Ganders sustained slight wounds. The attacking party, which was very strong, secured possession of one rifle as well as the bicycles. They escaped uninjured. A relief party from Dingle barracks, on hearing of the ambush, proceeded to the place and brought in their wounded comrades.’ Gallagher went to fetch help for his wounded comrades, he ran down, in the dark, a mountainside so precipitous that ‘even people born and reared locally would hardly risk travelling down it in daylight. How they were not killed I cannot understand.
1922 Feb 10 disbanded from RIC
1922 Mar 18 enlisted in Palestine Gendarmerie. Previously a Permanent Constable in RIC. #24
1923 Jul/Sep married in Wandsworth, Surrey to Margaret Talbot
He spent most of his Palestine career in the Motor Transport section, moving up from Ford tenders to armoured vehicles as the security situation steadily deteriorated. Initially he was based at Jaffa Urban and later transferred to Jerusalem, where he lived in the Greek Colony with his young family.
1940 Jul. Still a Constable, Gallagher was awarded the Colonial Police Long Service and Good Conduct Medal. The requirement was for 18 years exemplary service, so clearly his Gendarmerie service was counted.
Between November 1945 and 1 July 1946, the United Jewish Resistance killed 13 and wounded 63 Palestine Policemen. On 22 July 1946 the Irgun bombed the British administrative headquarters at the King David Hotel, Jerusalem, killing 90 people. Gallagher rose to become a First British Sergeant, left Palestine on his retirement leave on 1 September 1946 and was awarded a Second Long Service Award Bar in July 1947.
1963 Apr 16 He died in Surrey age 65. Peter was living at 27 Sutton Grove Surrey. and died at St Helier Hospital Carshalton Surrey. his son J Gallagher was present at death from Coronary Thrombosis.