The singer, asleep in Philadelphia, dreams of Glen Ullin church. The McLaughlins are preaching, and Irish heroes such as the Parnells and Dan O'Connell are present. His wife shakes him awake, and he realizes he is far from the old home
I must assume that the several clergymen mentioned here are local figures; I cannot find any clear historical references to any of them. The political figures are another matter. They include:
The Parnell Family - Charles Stewart Parnell (1845-1891) was leader of the Land League from 1879, and supported Home Rule for Ireland for the rest of his life. Imprisoned in 1881, he became an Irish hero, and from 1885-1890 he held the balance of power in the English parliament, but found himself distrusted by both sides and, eventually, discredited by a personal indiscretion (see "We Won't Let Our Leader Run Down").
Dan O'Connell - Daniel O'Connell (1775-1847), a crusader for tenant freedom, for whom see Erin's Green Shore [Laws Q27]
Brian Boru - an odd name in the list; Brian Boru was King of Clare from 976, and died in battle against the Vikings at Clontarf in 1014. Held up as an Irish hero, he was never actually King of Ireland, and did not fight against the Anglo-Normans, who invaded centuries after his death.
The Redmonds -- The date of the song here becomes important. I am guessing that it is a reference to John Redmond (1856-1918), who managed in 1900 to recreate Parnell's Irish coalition and restore the Home Rule campaign in the British parliament.
The O'Sullivans -- perhaps Sheamus O'Sullivan, a minor poet who wrote in support of Parnell, and/or Sean O'Sullivan, a minor leader in the 1916 Rising. - RBW