As Anon mentions above, you're probably thinking of Smithwick's with a Guinness head, rather than the other way around — Guinness has a creamier head, which I think is the reason why people order it.
It is not, by any stretch a Black and Tan — and yes, the name would be seen as offensive by most in Ireland and I have never heard ordered by locals (this is a sure way to spot a tourist) — which, as far as I know, is normally made with a larger or pale ale (Smithwick's is a red ale), and whose ratio is more like 50:50. It would look something like this:
Smithwick's with a Guinness head is just that — the proportion of Guinness in the drink is just the head, i.e. maybe a inch on the top of the glass. The pint of Smithwick's is poured as normal, with a small amount of Guinness added at the very end, which sits on top of the pint giving it a head of stout. Adding too much Guinness at this stage will cause the beers to mix, and will ruin the pint.
Can't find a picture of it, but it will basically look like a pint of Smitwick's with a creamer (look and texture) head, and taste as such.
As a barman in a country pub for a couple of years during college, I had a couple of regulars who used to order it. It's not particularly common, but neither is it unusual — any barman should know what you want and be able to serve it.