Jesus is life?

Even Gaelic has its urban myths. One of them is that 'S e do bheatha is supposedly Is E do bheatha, as in He is your life, i.e. either God is your life or Jesus is your life. Nice try, but no cigar.

It's true that if you dig a bit further back into history, you come across dia do bheatha in Old Irish (yes, fortunately they wrote things down), so we have an instance of CuChulainn greeting Fergus with Fuit! Día do bethu, a phopa Fergus, in the Lebor na hUidre. So, while this looks a bit like it might be invoking anthropomorphised omnipotent beings, there's an immediate problem. It's unlikely to be invoking the Christian pantheon because the Fianna didn't do Christianity.

Bearing in mind very similar Old Irish formulae, such as

• rotbia-su fáilte "to you will be welcome"
• rotbia in failti sunda againni "to you will be welcome here at us"

it is much more plausible that the origin of this phrase was rotbia do bethu "thou shalt have thy life" (also appears as rotfia do betha in a 15th century manuscript), with ro-t·bia being - no, not root beer - but the following:

• ro, an Old Irish preverb (a particle which may go before a verb), a form of do meaning "to(wards)"
• -t- a marker for the second person "you", so it's a little bit like having the modern dhut "to(wards) you" sitting before the verb. But, before you go down that route, it's not dhut per se, because that was duit/dait, in Old Irish
• -bia 3rd person singular future of the verb "be"; think of modern bi(dh) e.

The de is what it seems to be, the preposition "of" and bethu is, well, bheatha.

Over time, this would change quite regularly:

1. rotbia do bethu drops the preverb ro leaving us with
2. tbia do bethu which immediately simplifies tb- to just t- which weakens and slenderises it giving us
3. dia do bethu giving us
4. día do bheatha which is then re-analysed to
5. dé do bheatha which further reduces to
6. sé do bheatha and is then re-analysed, again, as
7. 's e do bheatha.

The development of tb to d is a little weird on the face of it but there's a well-attested parallel, at·beir "says it" similarly turned into a·deir.

It also fits in much better with a handful of derived phrases that sometimes come up in formal/fancy Gaelic, tha thu di-beathte and the stronger làn di do bheatha (or ur beatha of course) for "you are (very) welcome" (both in response to an expression of gratitude and to welcome someone to a place) which clearly does not invoke any 3rd person.

There are interesting glimpses of this elsewhere too, in more recent literature. Though a little tricky (because it's a translation rather than a native composition) the Gaelic Bible of 1902 (which was actually translated several decades earlier) has this phrase in its apparent native usage in Tobit 5:13 : An-sin thubhairt Tobit, 'S e do beatha, a bhràthair - and the context makes it clear this is not in response to someone saying thank you but quite literally welcoming Raphael to the place. There's another instance which even has the opposite. Setanta has just killed Culainn's dog and Culainn asks Setanta who he is and when he hears his answer, he responds with 'Se do bheatha air sgàth d' athar is do mhàthar ach chan e do bheatha air do sgàth fhèin as in, "well met for the sake of your father and mother but ill-met for your own sake".

It also very clearly appears in this function in Irish, like the famous song Óró sé do bheatha abhaile which is attested as far back as 1855 at the very least (it appears in George Petrie's Complete Collection of Irish Music and is glossed in one instance as Welcome home Prince Charley in Petrie and in general as welcome home in virtually all other glosses of this song.

So paraphrasing the idiom, it's really just a very Gaelic way of saying live long and prosper. 🖖