D'fhéach mé ar http://www.focloir.ie agus tháinig mé air seo: 'cosa crainn a chur faoi chearc'. Tá cuma an-suimiúil air! An bhfuil éinne in ann é sin a mhíniú dom?

To put legs of trees under a hen? I just don't quite get how that means to reinvent the wheel.

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The word crann can also mean wooden, as an adjective, so perhaps the meaning is that it's silly to put wooden legs on a chicken, since it already has legs, just as it's silly to start all over again re-inventing the wheel.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

There was a discussion about that one on Facebook recently.
I don't remember which group or what the consensus was.

I don't think 'cosa crainn a chur faoi chearc/na cearca' is a great translation of 'to re-invent the wheel'.

http://www.irishlanguageforum.com/viewtopic.php?f=28&t=4606&view=next

Foclóir.ie uses the same Irish expression for 'to promise someone the earth/moon'. That one looks a bit better, I think.

In Tipperary it was a fairly common expression to describe some one having all the answers no matter what the problem or question was. ''She'd put feet under hens for you" meaning she knew everything. I seem to recall the idiom being used in the novel ''An Béal Bocht'' by Myles na gCapalleen(Flannery O'Brien)

Anseo

Ní fada gur phléamar an nath anseo:
viewtopic.php?f=28&t=4790

Bíonn mé scaipí.