Hi
The index book for the DMP is:
Jim Herlihy, The Dublin Metropolitan Police: A Complete Alphabetical List of Officers and Men 1836-1925, Four Courts Press, Dublin, 2001.
He also wrote (in adition to his books on the RIC):
Jim Herlihy, The Dublin Metropolitan Police: A Short History and Genealogical Guide, Four Courts Press, 2001.
Unlike the RIC List, the DMP List gives the birth year and native Parish and County of the member, in addition to the DMP Registered Number. Herlihy also indicates where a member was Ex-Constabulary - i.e. formerly a Royal Irish Constabulary (RIC) member, or was later a member of the Garda (the police force in Southern Ireland after Partition - this force is still in operation).
Page 199 of the DMP LIst gives five O'Malley entries -
O'Malley, Austin, DMP 10023, born 1874, Aughavale, Co. Mayo
O'Malley, George, DMP 2263, born 1812, Frankford, Co. Tyrone; Ex-Constabulary
O'Malley, George, DMP 2332, born 1814, Westport, Co. Mayo, Ex-Constabulary
O'Malley, Martin, DMP 5659, born 1836, St. Michan's, Dublin City; Ex-Constabulary
O'Malley, Patrick, DMP 12226, born 1902, Kilmeena, Co. Mayo; Garda, died 1948.
The DMP Registered Number allows the actual member's record to be accessed at the National Archives (Dublin or Kew) or the Garda Museum (the LDS has this data too).
Further information on Ex-Constabulary members can be got from the RIC records - bearing in mind that the Herlihy RIC List only gives RIC Registered Number - so you might need to look up RIC membership entries for, say, George O'Malley before finding the correct one.
If children of the O'Malley member (son of a West of Ireland-born member) were born in the mid 1860s then there is only one member who might be the father from the West of Ireland - George O'Malley (born 1814, Mayo). Martin O'Malley (born 1836, Dublin) could be the son. However, bear in mind that in the earlier years of the force the records may not have been complete.
I hope this helps
best regards
Gerry McMahon
Dublin