A little more info, from T. Crofton Croker's "Popular Songs of Ireland" (1839). Sir Charles O'Donnell informed Croker in 1833 that

"Mr. Connell (the Johnny Connell of Garryowen) and Darby O'Brien (some versions have Harry, others Jerry O'Brien) were two squireens in Limerick, and, about the time the song was written, between the years 1770 and 1780, devil-may-care sort of fellows, who defied all authority: they were the sons of brewers; the former is still alive, and has, or had, until very lately, a large brewery in Limerick."

A "squireen" holds a small estate. Croker also quotes from a London weekly of 1822:

"The celebrated Garryowen forms part of the filthy suburbs of Limerick. The former character of its inhabitants is said to be well described in a verse of their own old song:

    In Garryowen we'll drink nut-brown ale,
    An score de reckonin on de nail ;
    No man for debt shall go to gaol
         From Garryowen in glory whu! [a yell.]

"Some years ago the Garryowen boys, headed by a young gentleman of respectable family, did what they listed in every department of heyday wildness and devilment : they were the half-terror, half-admiration of the surrounding communities. ...[T]he old
leader, to whom I have alluded, is now a most respectable
quiet citizen, about sixty, famed for propriety and urbanity of demeanour, and at the head of one of the most thriving mercantile concerns in the town. My antiquary (Mr. Geoffrey Foote) pointed him out and introduced me to him, the other day, in the streets ; and I futilely sought, in the grave and generous expression of his features, in the even tone of his voice, and in the Quaker cut and
coloured suit which he wore, for any characteristic of the former Georgie Robinson of an Irish Porteus mob. Neither age nor change of habits had altered the tall and muscular figure which, in the redolence and buoyancy of youth, must have been equal to any achievement of physical prowess."

Thomond Gate and Garryowen, Croker says, were on opposite sides of Limerick.

Croker's version of the song is identical to Lenihan's.

The words are easier to sing to the somewhat simpler tune of Aird's "Auld Bessy" (1788) than they are to the fuller modern version in "Harlequin Amulet." See the other current "Garryowen" thread for pedantic details.