After searching long and hard, I have still not really got anywhere, but I thought that it may be useful to post a link to a decent picture I found of the tattoo that was predominantly shown incase anyone sleuth skills are better than mine

:
http://img71.photobucket.com/albums/v215/s...rius_tattoo.jpgand
http://www.bellaonline.us/~bodyart/movie/azkaban/sirius3.jpgI actually thought that it may even symolise the dogstar or sirius but it only seems to half-work if you turn it upside down
http://ecuip.lib.uchicago.edu/diglib/scien...om_stars-1b.gifbut I did find something interesting (when following this dog-star' link to tattoos idea) at
1)http://www.crystalinks.com/inundation.html
QUOTE
It was up to the Egyptian priests, who attended to the calendar, to sight the first rising of Sirius. At the ancient temple of Isis-Hathor at Denderah, is a beautiful statue of Isis, located at the end of an aisle flanked by large columns. The statue was oriented to the rising of Sirius and priests would place a jewel in the goddess' forehead so that the light from the returning star would fall on the gem.
Isis is a symbol of rebirth so perhaps the tattoos could indeed be linked to rebirth, since a few other websites have also linked isis and sirius (the star that is

), which given the strong link between Isis and rebirth could suggest a further link between sirius and rebith (am I grasping),
So I started looking for Rebirth symbols that could be similar to tattoo, these include
1)Bullhorns (grasping at straws),
2) The ankh (it is sort of similar)
3) The [phoenix (ok this is a bit of a wierd one, but whenever I typed in rebirth symbols as a search the pcitures of phoenix that came up all had their wind out are curved upward like the tattoo) - ok perhaps not for that one
so no real strong links there
However, here is what Wikipedia said about Neptune (thus far the closest we have come to Sirius' Tattoo)
QUOTE
Neptune
Modern Western astrologers associate the planet Neptune () with illusion, deception, religions, spirituality, the mass media, music, drugs, extreme sensitivity, psychic phenomena and altered mental states. The discovery of Neptune in 1846 coincided with the discovery of anesthetics and hypnotism around this period.
For some astrologers, Neptune is the ruling planet of Pisces. Prior to the discovery of Neptune, Jupiter was considered the ruler of Pisces, and some modern astrologers consider Jupiter a co-ruler of Pisces. Some astrologers do not believe that Neptune rules any particular sign, even though they may use the planet in interpretation.
or perhaps the runic symbol of Algiz - which is for protection
QUOTE
Algiz: (Z or -R: Elk, protection.) Protection, a shield. The protective urge to shelter oneself or others. Defense, warding off of evil, shield, guardian. Connection with the gods, awakening, higher life. It can be used to channel energies appropriately. Follow your instincts. Keep hold of success or maintain a position won or earned. Algiz Reversed: or Merkstave: Hidden danger, consumption by divine forces, loss of divine link. Taboo, warning, turning away, that which repels.
see:
http://www.techno-feng-shui.nm.ru/Action/Algiz.giffor a picture
and
http://images.google.co.uk/imgres?imgurl=h...3D%26safe%3Doff for a proective shield made from it which also has the cross bars of the tattoo
so yeah I'm no further on - sorry, but I am still searching and at least I've managed to get rid off a few ideas that I had which is sort of progress in that I know what it is not 
and sorry that this post was full of links and quotes (I'll get rid of them if you want)