In "The Sniper" by Liam O’Flaherty, how would I write a journal from the point of view of someone who witnessed the gun battle?

First, I think that you would need to decide what sort of
a person your witness is.  Is this an outsider who would be coming in and would be
shocked by the violence?  Or is it someone who lives in the neighborhood and has seen it
all before and is jaded?  Does your witness support one side or the other in this
conflict?

Once you have figured this out, you have your
journal entry mostly written.  You need to give some details of what happened --
probably more details if your witness is an outsider (because they'd be more freaked out
and likely to record everything).  You need to show the incident's affect on the
witness's emotions, if any.  If your witness cared about who won, there would be a lot
of emotion, if not, maybe the witness would only comment  on how clever the "winning"
sniper was.

Remember that your witness does not know that
the two snipers were brothers ....