Green’s Dictionary of Slang

Bannagher n.

In phrases

beat Bannagher (v.) (also beat Banaghan, ...Banagher, ...Banaher, ...Bannigan, bang Banagher, ...Bannager, ...Bannaghar, ...Bannagher, ...wattle gum) [? name of a real story-teller who is surpassed by the current talker; f. the town of Banagher, a notorious ‘rotten borough’. To ‘beat it’ would be to surpass any extreme]

[late 18C+] (orig. Irish) to tell fabulous, fantastic tales; often with addition and Banagher beat the Devil.

In exclamations