Dear Marcel,

From Ancestry.Com, Justice Lemerce, which is listed on the Civil War Soldiers and Sailors site as an alternate spelling for Justice Lemire, is listed as follows:

Name: Justice Lemerce ,
Enlistment Date: 10 August 1864
Service Record:
Enlisted as a Private on 10 August 1864 at the age of 18
Substitute in Company C, 4th Infantry Regiment Minnesota on 10 August 1864.
Mustered out Company C, 4th Infantry Regiment Minnesota on 12 June 1865

From CWSS:
Justice Lemire (First_Last)
Regiment Name 4 Minn. Inf.
Side Union
Company C
Soldier's Rank_In Pvt.
Soldier's Rank_Out Pvt.
Alternate Name Justus/Lemerce
Notes
Film Number M546 roll 5

The age appears to matchup very well with your Eustache Lemire.

I cannot find a pension record for this man, do you have any more details?

Jim Martin