Green’s Dictionary of Slang

O’Reilly n.

[O’Reilly was apparently a real person, poss. a trade-unionist on the Liverpool docks]

used as an intensifier in general excls. as below.

In exclamations

blimey O’Reilly!

[1910s+] a general excl., presumed to be an intensifier of blimey! excl.

blind O’Reilly! [1910s+]

1. a general excl. of surprise, excitement etc.

2. used as an intensifier.

God blind old Reilly!

[1940s–50s] a general oath of annoyance, incredulity, etc.