Bhelieve you the Universals!
Irish (and the other Celtic languages, where they haven't lost it) has two really eerie features. One, conjugation is done almost exclusively through initial and internal mutation, rather than than word final. Second is the word order, VSO (Verb-Subject-Object).
From an Indo-Europen point of view, these features are both very odd. In Indo-European the word order traditionally is considers SOV, though some argue that it's SVO, either way, it's definitely not VSO. Secondly, Indo-European is heavily reliant on suffixation (and internal mutation to some extent) to conjugate anything from verbs to nouns.
The interesting thing is. In SOV languages, we generally find suffixation, while in VSO languages we find prefixation. These are so called 'language universals'. I'm not a big fan of these universals, because they're disproved quite often, but right here it's not uninteresting.
Another famous VSO language is Arabic. Arabic makes use suffixation, but mostly relies on prefixes and internal mutation for the greatest differentiation. Now, the development of Irish initial mutation was a perfectly normal sub-phonemic process, which later became phonemic and grammatical. The great question is of course. Did these forms become grammatical because the word order shifted to VSO. Or did these initial mutations give rise a VSO word order.
It's as if some invisible force in our minds seems to have moulded this language into the 'universal' for no compelling reason except that it's not unusual to happen. But why? Very very interesting indeed.
Sorry I can't give any definite answers on this, it's currently far to mysterious, but I thought it would be worth mentioning
Comments
I don't see why we have to decide between SOV or SVO for PIE. Why can't it be both? There are many languages out there that change word order depending on things like mood or aspect of the verb. Of course, there is always a default word order along with some marked word order(s), but nonetheless word order is not unbending or static.
In English questions, we all know that the word order changes such that the verb is first, although in Modern English this is done with an auxiliary ("Do you play?") instead of the fun old fashioned way of using the verb itself ("Playest thou?"). So English is SVO with a little VSO to spice things up. In German, verbs in declarative sentences are pushed into first position before pronouns under certain conditions ("du kannst"/"aber kannst du"). I believe the Etruscan language, that I obsess so much over, moves the word order around too depending on focus and transitivity, but I'm trying to stay one step ahead of those darned Etruscologists so don't tell anyone, 'kay?... Ssssssh! :)
With PIE, my memory might be hazy because I dabble in a bunch of subjects at once but I recall there being something about a relationship between the selection between SOV/SVO and [-/+ perfective]. The unmarked word order would have certainly been SOV (and would have been the prevailing order in Pre-IE), but SVO must have already been prevalent in PIE itself even before it had spread outwards.
Now, it seems to me that as PIE spread westward into Europe, SVO was gaining popularity at the expense of SOV (which makes one ponder about possible features of pre-IE languages that existed beforehand that could have influenced that change). By the time PIE languages reached Western Europe and into Spain, SVO would have gained strong upper hand already. So any areal influences with Berber or Phoenician could easily set western PIE dialects over the edge and into the VSO end of the syntactic spectrum, no?