The 'have' of possession

I don't usually post to sci.lang, because I'm not a linguist, but this
topic is one that needs expert input. I hope nobody minds the cross-post
to the newsgroup I normally inhabit.
Almost all European languages have a "have" verb to indicate possession.
(And has other uses, but that's a separate topic.) The Irish language is
an exception, in that it lets a preposition do the job of a verb. The
equivalent of English "I have an apple" is "Tá úll agam", literally "Is
apple at me".
Scots Gaelic is similar (Tha ubhal agam), and so is Welsh (Mae gen i afal).
And so is Russian. The Russian for "I have an apple" is "у меня есть
яблоко", literally "at me is apple". Apart from word order, this is
identical to the Irish example.
This bothers me. What should (most) Celtic languages and (some) Slavic
languages share a feature that is not found in the many languages that
sit geographically between them?
My question: does this suggest that the Slavs and the Celts were in
contact at a critical time of language evolution?
An alternative possibility, I suppose, is that this used to be a
standard feature of IE, one that most of the successor languages
eventually lost. But that sounds less likely to me.
--
Peter Moylan                         http://www.pmoylan.org
Newcastle, NSW