1889 Sep 26 Born Dublin
1901 census at Killeen Road, Rathmines, Dublin
1911 census at St. Patricks Road, Wicklow. He is a "pressman"
1911 Apr 4. London Gazette reports - Census Office, Ireland: Temporary Male Clerks, Dermot McMahon Byrne
1915 Enlisted in ASC and was stationed in Hants.
1915 Mar 17. Gazetted 2nd Lt in RFA. The undermentioned to be temporary Second Lieutenants. Dermod MacMahon-O'Byrne.
1915 Jun Lands in Mesopotamia
1916 Mar. County Meath Roll of Honour. With the names, regiments and other interesting particulars of the men from Meath who joined the army to date.Also statistics of population, and record of the work done in Meath in connection with the war. Specially compiled by A.J. Horneck. Journalist Navan. The Leinster Leader Ltd, Naas. Published about March 1916.
D.McMahon O’Byrne – Lieut. Royal Field Artillery now in Agra, India. Enlisted in Army service Corps stationed in Hants; gazetted to R.F.A. March 1915. Resided in Navan for several years; occupation, Journalist; native of Rathmines; formerly in I.N.V.
1917 Jul 2. Second Lieutenant Dermott McMahon-O'Byrne, Royal Field Artillery, attached 25th Cavalry (Frontier Force). Dated 2nd July 1917, but to rank from 17th December, 1916.
1918 Marries in Chorlton, Lancs to May Rowlinson
1919 May 25. RFA. Lt. D. MacMahon-O'Byrne, from Ind. Army, to be temp. Lt.
1920 Sep 27. Joined ADRIC with service no 628. Post F Coy
1920 Oct 21. Promoted Section Leader and i/c coy stores
1920 Dec 1 Took part in raid on Edward O'Connor's house (see under T/Cadet Frecker)
1921 Feb 12 Promoted DI.3. Register records he was an Intelligence Officer which I assume was at this promotion
1921 May 13 In charge of raids in Dublin
1921 May 14 In charge of Raids in Dublin
1921 May 16. On a raid in Dublin
1921 May 19. On raids in Dublin
1921 May 25 Was at the Custom House Incident
1921 Jul 2. Relinquishes his IO appointment with F Coy
1921 Aug 26 Becomes Permanent Cadet.
1922 Apr 21. Pensioned.
1927 Nov 11 married Louie Poole in Bucklow, Cheshire
1931 Aug 26. 93rd (E Lancs) Fd. Promoted from Lt to Capt.
1932 Apr 24. 93rd (E. Lan.) Fd. Bde.—Capt. D. MacMahon- O'Byrne to be Maj.
1935 -1940 living in Accrington, Lancashire (phone books)
1938 Nov 26. The undermentioned offrs. are transferred from 93rd (E. Lan.) Fd. R., R.A., retaining rank and seniority: — Majs. D. MacMahon-O'Byrne
1946-1970 living 68 Manley Rd, Mossside, Manchester
1948 Sep 1. The undermentioned Majs. having exceeded the age limit, retires retaining the rank of Maj.:— D. MACMAHON-O'BYRNE, T.D. (32436).
1975 Jun Died in Trafford, Cheshire, England