"correct! your opinion....not indisputable fact,"
It is pretty well established that it was common practice for 'events', political and social, to be recorded in song.
We've been digging up information on local songs since we moved to Ireland fifteen years ago, and have been recording such songs for at least thirty years..
These songs, as far as we've bee able to establish, were made, invariably by people who seldom put their name to them, and they became part of local history - many of them all-but disappeared when the particular events faded from local memory.
They included political events, emigrations, weddings, murders, accidents..... there's even one about a respected priest moving on to the next parish..... they were part of folk creation.
Quite often the same event was commemorated by several songs - locally there were four songs made about The Rineen Ambush (a Black-and-Tan convoy ambushed a few miles from here), the same number about The West Clare Railway, at least a half-dozen about the sinking of a ship off the coast in 1911, when local fishermen rescued the crew in curraghs.
The closest to 'Barry' were two songs entitled 'Mac and Shanahan' describing the arrest, torture and murder of two young men suspected of political involvement by Black and Tans.
Songs like these became part of the struggle for independence, and they also served as an expression of the local fights for independence - they are part of social history.   
Songs like Kevin Barry came about at a time of intense national struggle and they became part of that struggle.
They sometime took 'lament' form, as here, but they were also triumphant, angry, funny.... every emotion that is associated with the political and social life of the times they were made.   
They were sometimes produced in songbooks and were sung at political rallys and demonstrations, others just survived in the memories of those who were around at the time - there are hundreds of them.
The broadside trade thrived on gathering up some of these songs and selling them; in some cases they were deliberately made and printed to deliver a specific message.
The most well-known case of this was 'Patrick Sheehan (aka The Glens of Aherlow) - family evicted by landlord, parents starve to death at the roadside, son goes into the workhouse and then joins the British army and is blinded at Sebastopol, his disablement pension expires and he eventually ends up begging on the Dublin streets.
The song was based on an actual event and was written by well-known Irish writer, Charles Kickham, who actually intended it to be used as a disincentive to recruitment, but it ended up being instrumental in changing Irish law on service pensions.
I would be interested to learn if you actually know anything about these songs - there's tons of information about them, I would highly recommend Georges Zimmermann's Songs of Irish Rebellion.
Your failure to respond to one single point I have made about the political situation in Ireland pertaining at the time these songs were made suggests that you are drawing your conclusions from your own political leanings, but I would be happy to be proved wrong on this and add your information to our own researches.
Jim Carroll