Jim, let's leave WW2 to another thread.
Also, we are all aware that WW1 was bad. No need for a long emotive post.
The issue of why men volunteered you left untouched.
Young men like yours, with a yearning for adventure, have always been drawn to the army. Peace or war.
We have to explain why a million or so from these islands volunteered specifically for this war.
They mostly left steady jobs and loving families.
Families and friends and lovers encouraged them. (We don't want to lose you, But we think you ought to go.)
Why?
For adventure?
No. It had to be a cause fervently believed in.

The armies of a brutal, military regime were sweeping through France and the Low Countries with no guaruntee that they would stop at the Channel.
I suggest that alone would have been enough, without the reports of atrocities.

Were there allied atrocities against civillians that were suppressed?
No. The allies were fighting a defensive war against invading armies on their own territory.
German atrocities may have been exagerated, but the happened.
6000 Belgian civillians in the first week.
You can visit the towns and read the names.
http://www.firstworldwar.com/source/germanatrocities_usreport.htm
http://www.historyofwar.org/bookpage/lipkes_rehearsals.html
Why did Irish men volunteer?
No brainer.
Look at the Irish newspapers of 1914.
You will not find a column inch about implied promises of home rule.
Sinn Fein, by its utter failure, showed that there was absolutely no interest in it.
But you will find acres of print about the threat from the evil Hun.