Hello. I was wondering if anybody could help me by translating
"May the sun guide you and the moon watch over you"
into Irish Celtic?
Please and thank you.

The term "Celtic" is not used in that way, but I assume you meant to say "Irish Gaelic", which is nowadays usually just called "Irish".
Here's a fairly literal translation of what you want:

Go dtreoraí an ghrian thú agus go dtuga an ghealach aire duit
or, with the word agus ("and") abbreviated in a way which is very common:
Go dtreoraí an ghrian thú 's go dtuga an ghealach aire duit

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

Hi!

There's no such thing as "Irish Celtic." I assume you mean the Irish language. That's simply called "Irish" (sometimes, in programs outside of Ireland, "Irish Gaelic."

Here's my first pass at your translation, but please wait for more input, and at least three people to be in agreement on a translation, before proceeding with it:

Go dtreoraí an ghrian thú is go bhfaire an ghealach thú

That would be if you wanted it to sound as if you're only addressing one person. If you want to address it to more than one person, it would be:

Go dtreoraí an ghrian sibh is go bhfaire an ghealach sibh

Wait for more input, please.

Redwolf

Oops! Missed this while I was typing! What I posted isn't a correction of Caoimhín's, mtaylor1996...just another suggestion.

Redwolf

Alternatives for "May the sun guide you ..."

Go dtreoraí an ghrian thú ...
Go stiúra an ghrian thú ...
Go raibh an ghrian mar threoir agat ...
Go dtuga an ghrian treoir duit ...

and for "... and the moon watch over you"
... agus go bhfaire an ghealach ort
... agus faoi choimirce na gealaí go raibh tú
... agus go gcoinní an ghealach súil ort
... agus go gcoimhéada an ghealach súil ort
... agus go dtuga an ghealach aire duit
... agus go n-amharca an ghealach i do dhiadh

Thank you all for your input and speedy replies. It is much appreciated!