If you get hold of a 1960's Teach Yourself Irish book, you'll see that counting by tens and twenties is what it taught. The traditional counting method is also still used in Scottish Gaelic, for the most part.

One other note is that people sometimes don't realize that dáichead is just an abbreviation for d[h]á fhichead.

On the old site, one of the more active (and strident) members once lectured us all that it's a myth that counting by 20's was the original way to count in the Celtic languages, and that it was actually adopted at some point from elsewhere, though I forget what he said was the actual time period when it was adopted, and whether he explained why it happened.

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

I don't think we can know. Anyway it must be very old since all Celtic languages do count by 20's and they have split into several groups and languages a very long time ago. More than 2000 years ago... and we don't know what other language could have influenced Old Celtic. The origins of 20-basis counting is pure hypothesis, and probably impossible to verify...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Tá sé suimiúil; ní raibh sé sin ar eolas agam.

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