I'm thinking of getting the proverb "When death comes he won't leave empty-handed" tattooed in Irish. I know that in Irish it uses the relative form which isn't found in the Caighdeán.

I'm wondering which is the more correct way of spelling it, the two options I've found are:

"Nuair a thiocas an bás ní imeoidh sé folamh"

"Nuair a thoicfas an bás ní imeoidh sé folamh"

So basically the decision is whether to include the f.

I know in the Caighdeán this would probably be said as:

"Nuair a thiocfaidh an bás ní imeoidh sé folamh"

Go raibh maith agaibh as ucht cabhrú liom!

thiocfaidh -> thiocfas

seconded

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

Go hiontach! So "a thiocfas" is the preferred dialectal form?

Go raibh míle maith agaibh

In the dialects that use it, yes, it's the preferred form for the future relative.

Which is Connacht and Ulster dialects?

As far as I'm aware, yes. Though Connacht does use a different format for the present than Ulster.

Cheap mé é sin. (e)anns (Connacht) vs. (e)as (Uladh).
Go raibh maith agat

In the present tense, not all verbs have the -as ending in Donegal: certain irregular verbs don't have it.
Eg. : an duine a thig (I don't think you'd say "a thigeas")...

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