Not sure if Mrrzy is really asking the question or tongue in cheek. But just in case, it's "join" in the sense of "become a member of, associate with" rather than join them to each other.

The "united men", referred to in the added stanza, were the United Irishmen - Catholic, Protestant and Dissenter.

Oulart hill was the site of a battle during the 1798 rebellion, near Boolavogue the home of Father Murphy, who initially preached sermons condemning the growing disorder in his locality, but got caught up in the rebellion when the militia failed to honour an agreement which he helped to mediate with a view to calming the situation.

The bicentenary commemoration in 1998, helped no doubt by the outbreak of something like peace in Northern Ireland, was noteworthy for the serious effort made in the South to correct the traditional oversimplified monolithic catholic/nationalist/republican narrative of '98.