What's the best version of this?

Is neach beó mé ó tá cumas smaoinimh agam?

I don't like things like "táim-se ann", which strike me as too simple for philosophical language.

Deinim machnamh, agus ciallaíonn san go bhfuilim ann?

Machnamh a dhéanamh, is ionann san agus bheith ann.

Meabhraím, dá bhrí sin is rud dáiríribh me.

I take your point about the simplicity of the language used in the expression, but that's how existence is generally expressed in Irish, eg. an bhfuil Dia ann? - "does God exist?"

I find quite often when translating from English to Irish, and the English uses complex philosophical or scientific terminology (itself typically appropriated from Latin or Greek), that the natural way of expressing it in Irish seems somewhat imprecise or simplistic. I'm also led to believe that this is quite a common phenomenon in languages other than Irish. That technical language can feel unusual or clunky and so, where necessary, complex concepts are expressed in terms which may seem simplistic by comparison to those used in English. This may not be the case for other major languages like French, German, Arabic, etc., in which a significant amount of scientific and philosophical literature has been written over the centuries, but I suspect it is the case for the majority of the less widely spoken European languages.

In this case, though, I don't think there's anything complex about even the English. The most obtuse word in the phrase is "therefore". My suggestion would be to keep it simple:

machnaím/smaoiním, táim-se ann mar sin de.

OK sounds good.
The "I am" sounds odd in English, as usually you say "I am something" - I am a man, I am tired etc. But in the Bible where God calls himself in Exodus "I am that I am", Peadar Ua Laoghaire has simply "is mise atá".

Exodus 3:13, 14:
13 Duairt Maois le Dia: Féach, raghadsa ag triall ar chlaínn Israél agus déarfad leó: Do chuir Dia úr n-athrach mise chúibh. Dá n-abraidís liom: Cad is ainm do? Cad ’déarfad leó? 14 Duairt Dia le Maois: Is mise atá. Duairt sé: Seo mar adéarfair le claínn Israél: An tÉ atá, is é ’ chuir me chúibh.

The Douay has:
[13] Moses said to God: Lo, I shall go to the children of Israel, and say to them: The God of your fathers hath sent me to you. If they should say to me: What is his name? what shall I say to them? [14] God said to Moses: I AM WHO AM. He said: Thus shalt thou say to the children of Israel: HE WHO IS, hath sent me to you.

The footnote to v 13 says:
Is me an bhith féin, síoraí, ann uaim féin, gan bheann, gan teóra, gan tosach gan chrích gan athrú; an tobar as a dtagann gach bith eile.

The Douay footnote is:
"I am who am": That is, I am being itself, eternal, self-existent, independent, infinite; without beginning, end, or change; and the source of all other beings.

There is already a thread with this topic: viewtopic.php?f=28&t=3672

"too simple for philosophical language" - Well, it is "I am" in English, "bin ich" in German, "sum" in Latin, very simple, it was Descartes’ intention to make it simple.
That is why I’d make it even more simple in Irish: táim - and even leave out "ann".
(That is kind of incomplete, but mise atá, is É atá is incomplete, too. And "ergo sum", "therefore I am" and certainly "darum bin ich" is incomplete as well - and so a generalisation of existence.)

Táim mar smaoiním.

Or leave out "tá":

Ó smaoiním is ann dom.

This is nice.

I like the simplification to simply "Táim", especially given Peadar Ua Laoghaire's use of "is mise atá". However, I don't think it's quite right using mar like this.

Táim mar smaoiním strikes me as a translation for "I am because I think", which isn't quite the intended meaning of the phrase. The intent is to express "I know I exist because I am capable of thinking" not "I exist as a result of thinking". Deliberately misunderstanding this distinction is the basis of the "Descartes walks into a bar" joke:

Rene Descartes walks into a bar and orders a drink.
When he finishes his drink, the bartender asks him if he would like another.
Descartes replies, “No, I think not,” and disappears in a puff of logic.

In other words, Táim mar smaoiním suggests to me that I would cease to exist in any capacity if I were not presently thinking. This is not necessarily true. Not thinking might mean that I were dead or comatose, but it wouldn't necessarily mean that I don't exist in any capacity. Existence is not dependent on thought, but one's own existence can be affirmed to one's self by the recognition of thought.

I am not much familiar with philosophy. And I'm not an idealist, rather a materialist. I can easily exist without thinking.

But yes, "mar" is too much a monocausal conjunction. Something more in the sense of therefore or ergo is needed (and closer to the original).
I cited somewhere else but I don't know the source: Déanaim smaoineamh dá bhrí sin táim ann, so:

Smaoiním dá bhrí sin táim.

I would say:

Smaoiním, agus dá bhrí sin, táim.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal