What is the origin of "Couldn't hit a cow's arse with a banjo"?

I believe it is an Irish slang phrase.

You couldn't hit a cow's arse with a banjo (phr): bad aim, woeful hurler/darts player/soccer player etc.

Below are some Google findings which make me think this way:

• Dublin Slang
• Everyday English and slang in Ireland
• BBC - Northern Ireland - Voices
• Irish Vocabulary, Everyday English and Slang on Irish Celtic Jewels
• Everyday English and Slang in Ireland (scribd.com)

I also came accross an interesting forum thread which mentions this phrase. Here is an excerpt:

Jackie Milburn: - My dad used to say "son, yer couldn't hit a cow's arse with a shovel"

Juande the Spurrier: - There is a shovel that is called a banjo, because it's shaped like the musical instrument. Seems a farmer might be more likely to have a shovel than a banjo.

With further research, I found out that banjo is a shovel type indeed.

Banjo (as a shovel):

Banjo (as a musical instrument):

I hail from the Highlands of Scotland and this phrase has been in common use throughout my life. It was a favourite of my dad's. I'm 56 now so it's been around at least since the 1950s. Though, I must point out that, in front of my mum he always used the form backside rather than arse.