I've noticed many (all ?) Scottish Gaelic speakers called McDonald in English use "Domhnallach" as their surname in SG, not MacDomhnaill as one might expect.
Is their any particular reason for this?

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(SL)(GM)

Several Scottish clans have that same practice, and actually both MacDhòmhnaill and Dòmhnallach are in use (note the spelling changes). I wouldn't know which is more frequently used.

Here are some similar situations, some with a variant adjectival ending which looks like a Gaelicization of a -son ending:

Ferguson: MacFhearghais / Fearghasdan
Finlayson: MacFhionnlaigh / Fionnlasdan
MacDougall: MacDhùghgaill / Dùghglach
Morrison: MacIlleMhoire / Moireasdan
Munro: Mac an Rothaich / Rothach
Robertson: MacDhonnchaidh / Robasdan

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

Interesting stuff.
Would the same individual use both 'Dòmhnallach' and 'MacDhòmhnaill' in different circumstances or would they consistently use one or the other?

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(SL)(GM)

I don't really know. I found this on Wikipedia, but it only explains the fact of the alternate name, not how it came about:



Later on in the article it mentions that the longer formal forms are often not used in everyday speech, so for:


That doesn't explain the adjectival form being used, but it is probably also the result of less formality in everyday speech.

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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 hadn't thought to search Wikipedia for it. I've found the 'Scottish Gaelic personal naming system' page.
Thanks for that.

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(SL)(GM)