Origin of water under the dam

I'm trying to find out the origin of "water under the dam." Was it an expression used in the nineteen twenties and was it considered an Irish expression?

It is a malaphor (mixed metaphor) and, according to the following source, a common one:

That’s water under the dam:

This is a commonly used malaphor, blending two idioms – “water under the bridge” and “water over the dam” – that contain the same word (water) and have the same meaning (past and unchangeable events). Add two words that both describe direction (over and under), and two structures that are in or over water (dams and bridges) and you get a subtle mix-up.

(Malaphors.com)

The expression is probably more commonly used in speaking, but here are a few usage examples in writing:

From "How can I fix it?" (2001):

Jackson said that she did feel bad about making the accusation, but it was now water under the dam.

From "Devil's Waltz: an Alex Delaware Novel" (2003):

Staring at the sodden cloth with surprise and revulsion. She dropped it as if it were lice-ridden. “Forget it,” she said. “Water under the dam.”

I think this is an accidental mix of "water under the bridge" and "water over the dam", which is a less common variation of the former.

The earliest form of the phrase is water has flowed under the bridge appearing in 1858 according to OED. They also suggest comparing it to the French expression "il passera bien de l'eau sous le pont", lit. ‘much water will pass under the bridge’ (1842 or earlier).

water under the bridge (also over the dam, under the dyke, under the mill, etc.): used in various expressions referring to the passing of time, or (in later use) to suggest that past events have been forgotten or are not worth bringing up or discussing.

1858 J. Kavanagh Adèle II. xviii. 288 Ah! water has flowed under the bridge, as people say, since those days.