https://www.ancestry.com/mediaui-viewer/collection/1030/tree/34820597/person/172255435927/media/bfcb2eaa-fc53-4c41-a641-1110753ed68c

Several online trees give the birthplace of the 7 Apr 1803 James JOHNSTON son of Nicholas and Margeret (BEATTY) JOHNSTON as the parish of "Domolovin" in County Fermanagh, Ireland. This is erroneous and comes from the incorrect confusion of the James JOHNSTON who joined the 92nd Foot Soldiers in 1821 with the 7 Apr 1803 James JOHNSTON. In fact, the military record shows that the James JOHNSTON who joined the 92nd Foot Soldiers had first enlisted in 1819 at age 18 with the 58th Regiment, so that he was born in 1801 and not 1803 and thus is a different James JOHNSTON than the 7 Apr 1803 James JOHNSTON.

The military record is in the Ancestry "UK, Regimental Registers of Service, 1756-1900" Collection, image 128 of 224, numbered page 137, for the 92nd Foot Soldiers:

If you look at the "e" in Fermanagh and the letter right after the "D" in the parish name, they look the same to me.
Similarly, if you look at the "rm" in Fermanagh and what is after the "De" in the parish name, the parish name could be either "m" or "rn".
So, instead of DOMOLOVIN, my reading is DERNOLOVIN or DEMOLOVIN. Either way, there is no such townland or parish in townlands.ie for Fermanagh. So, the location of this place is a mystery.
If the person writing it down wrote the spelling that they thought that they heard, then the way it was said by a native and the way it was heard by a native of some other place could have resulted in a distortion of the sound heard.

Since all of the men had prior service (see below) in the 58th Regiment, ending 12 Dec 1821 (the day before), it looks like the absence of an entry in the "By whom enlisted" column indicates that they were either re-enlisting in the 92nd Foot Soldiers or that the 58th Regiment had been merged into the 92nd Foot Soldiers.

The From/To specifically says the period shown is "deducting Service prior to the Age of 18 Years".
The "Age at enlistment" column seems to indicate the age at initial enlistment. The man on the prior line enlisted at age 16 and is shown in the prior service column with service staring 7 Jan 1812. So, that man was born in 1794, if the 7 Jan 1812 starts when he turned 18.
Using the same reasoning, James Johnston was born about 1801 if he enlisted at age 18 and his prior service began 19 Nov 1819. The calculation of the birth year of the soldier has to be taken by subtracting the "Age at enlistment" from the date in the "From" column and NOT from the Attestation Date column.
The family trees on Ancestry give the birth of James son of Nicholas and Margaret (Beatty) Johnston as 7 Apr 1803 (although I have yet to find the source for that very specific date).
So, my tentative conclusion is that this military record is for a different James Johnston, born at DERNOLVIN/DEMOLOVIN in Fermanagh about 1801 and not the one born 7 Apr 1803.
Thus, we the association of DERNOLOVIN/DEMOLOVIN/DOMOLOVIN with the 7 Apr 1803 James Johnston, son of Nicholas and Margaret (Beatty) Johnston is an error.