I'm trying to find an accurate translation of a lyric for a Ben Howard Song

The Lyrics goes;

"I will become what I deserve"

Any advice would be gratefully received.

That's hard to say in Gaelic in a short phrase, and would probably have to end up as something like "I will become the man whom I deserve to become". However, to keep it short you might use the following:
Èirichidh dhomh mar a tha mi airidh
which means basically: "It will befall me as I deserve".

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

Thanks for that!

I don't mind if its not word for word, I appreciate that would be very silly,

I will look out for any corrections/suggestions!! But tihs is a great start!

Dwelly shows "airidh" being used with "is" rather than "tha". http://faclair.com/ViewDictionaryEntry.aspx?ID=F63568FE1930E679E35051925F8227C9http://faclair.com/ViewDictionaryEntry.aspx?ID=D1593FCF03DDF305874896F9BE5E4A9B

...mar is an airidh mi, perhaps?

The Faclair Beag entries show both "tha/bha/etc" and "is/bu", so it looks like you've got a choice. That said, with "tha", there's also usually an "air"... Èirichidh dhomh na tha mi airidh air or something, maybe.

(But this is me in dictionary mode... "airidh" isn't a word I'm familiar with anyway.)

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A language belongs to its native speakers, and when you speak it, you are a guest in their homes.
If you are not a good guest, you have no right to complain about receiving poor hospitality.

I'm no expert either, but I think the preposition air comes into play when there is mention of the thing of which one is worthy, as in:

Tha sibh airidh air moladh
You are worthy of praise [or: being praised]

but your suggestion still makes sense, and I like the na tha approach (I hadn't thought of that):

Èirichidh dhomh na tha mi airidh air
It will befall me as I am worthy of it [befalling me]

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

Yes, but if you have a look at the usages in the Faclair Beag, the "tha" variants all have a focus on the thing one is worth of, and the "is" variants are more about the worthiness or the worthy party.

Yeah, my only problem is that I'm much happier with the "air" before the verb (as occurs in most Scottish dialects) but I don't think this works with the relative pronoun "na" (=what), only noun+relative conjunction "a" (=that).

I suspect that doubling the pronoun "na ... air" is acceptable, but I wouldn't stake my life on it. If writing an essay to be marked, I'd possibly hedge it and go for an explicit noun + "air a": "..an rud air a tha mi airidh", but recognising that that looks really quite unpoetic, I'd change "an rud" to "a h-uile sion". But then that's starting to move further from the original meaning.

_________________
A language belongs to its native speakers, and when you speak it, you are a guest in their homes.
If you are not a good guest, you have no right to complain about receiving poor hospitality.