Hello, I am looking for a phrase that represents acceptance of what cannot be changed - as in "Come What May" or "Whatever will be, will be" or perhaps a more poetic, proper Old Irish phrase.

Thank you & have a beautiful day

For - come what may - I suggest: Is cuma cad a tharlóidh. This is my fate is - Seo í mo chinniúint, and - There's no getting away from fate is - Ní choisctear cinniúint. I've found a couple more phrases but they'd be too long for a tattoo such as - As sure as the sun rises tomorrow - chomh siúráilte le héirí gréine amárach - and - I will go whichever way (the) fate will take me - Rachaidh mé cibé bealach a dtabharfaidh an chinniúint mé - but wait for confirmation and suggestions from the others.

Thee are a number of expressions which use the word dán ("fate") in that context, so perhaps something like:
Pé rud é atá i ndán dom
Whatever is fated for me.

If you don't mean it as an expression about your own personal situation, then perhaps:
Pé rud é atá i ndán duinn
Whatever is fated for us.

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

Thank you for your responses! I really do appreciate it and I love your suggestions for "whatever is fated for me" and "this is my fate" :-) Here is a list of phrases that I have found, but not received any sort of consensus on:

cibé rud a tharlós -

Cibé rud a tharlóidh

Cibé rud a tharlaiónn

Cibé ar bith a tharlós

Cibé ar bith céard a tharlós

Tarlóidh an méid a tharlóidh

An rud a bhíonn, bíonn sé.

Tar cad is féidir

Pé rud a tharlóidh

What do you think?

You mentioned Old Irish sayings, here are some from the database of OI and Middle Irish texts:

Bíaidh do·berad ar ndee 7 ar dtoicthe dúinn - "We will have whatever our gods and our fate bring us."

Ni heidir dul sech an cinneadh (St. Ercuil 745) - "There is no avoiding destiny"

Ón chinneadh ní théid gach ní (O'R. Poems 1964) - "Everything doesn't go from fate" (ie. nothing can avoid fate). Ní théim óm' chinneadh / Ní théim óm' chinniúint / Cha dtéim óm' chinniúint could be modern renditions.

Ni fetann tiachtain ri toicthi - "He may not go against fate" (TTebe 3719), which would be written in modern Irish as Ní fhéadann sé teacht le toice "he cannot come with fortune", and subsequently, Ní fhéadaim teacht le toice for first person, but it is today more like "I cannot come with/accept fate" which is the opposite to what you're looking for.

For modern stuff, I've seen:
Is beag ár gcumhacht ar ár gcinniúint, meaning "we have little power over our fate/destiny", which would be Is beag mo chumhacht ar mo chinniúint in the singular.

Maybe:
Tugaim isteach dom' Thoice "I give into/yeild to my Fate/Chance"

Gabhaim leor le mo Chinniúint - "I accept my fate"

Is fearr glacadh ná gleic - "Acceptance is better than a struggle/wrestle"

Or maybe for a more Game of Thrones vibe ( ) go with something like Valar Morghūlis (All men must die), like accepting the inevitability of something:

Is dual dúinn uile an bás / Is dual don uile dhuine an bás - death must come to all of us / to everyone (.i. it is natural for us all to die)

Níor bhlais an bia nach mblaisfidh an bás - All men are mortal (lit. None who has tasted food shall not taste death)

I do like "Ni heidir dul sech an cinneadh" (St. Ercuil 745) - "There is no avoiding destiny" Just to verify for complete accuracy though, should there be any acute accents or dot of lenition?

Thank you everyone for helping me in my journey. I am very grateful <3

In standard Modern Irish, it would be: Ní féidir dul seach an gcinneadh. Seach as a preposition isn't really used today though, meaning "beyond/past", but it used to govern the accusative so an urú (the g in this case) is appropriate. Or for Ulster dialect, Ní féidir dul seach an chinneadh (lenited c, alternatively written with dot over c).
The original sentence is middle Irish, where no single standard was employed, so a standard modern or old Irish form probably works better.
One other note, cinneadh today is more like a decision or determination (which I guess fate/destiny is ), so technically the sentence could be read as "One cannot go beyond the decision" or "One cannot avoid/go against the determined action", but I think using seach implies "cinniúint" (the more common form of destiny/fate) moreso than "decision"... Just to bear that in mind.