I love the historical stuff and researching old tobacco has been very interesting.
I'd say your bar probably dates to 50's or 60's or maybe even pre-war, who knows?
The font looks Deco, like 20's or 30's !!! Judging from the picture here,
you can see how different a packet looked in the 70's
http://www.flickr.com/photos/sludgeulper/4682633771/
Even later it was probably sold in pouches, the A.I.T.S Tobacco Index by McGahey the tobacconist
circa 1995 lists it, but the description is a very brief "A full strength tobacco"
http://www.daft.de/uploads/Download/aits.pdf
Japan Tobacco bought Gallaher in 2007 for $14.7 billion.
Here's a really good read about the founder of it all, Tom Gallaher
http://archiver.rootsweb.ancestry.com/th/read/SCOTCH-IRISH-CULTURE/2006-06/1150200004
...more info
http://www.nmni.com/um/Collections/Collections-highlights/GALLAHER-S-TOBACCO
...inside info from a gent named David Mercer, who worked for Gallaher in the 60's
scroll about halfway down and there's a wealth of knowledge
http://futureobservatory.dyndns.org/2010.htm
...this is a great discussion too, fun to read
http://www.belfastforum.co.uk/index.php/topic,37355.0.html
...and you can go back in the wayback machine to see their old website,
not much there though
http://web.archive.org/web/20020328035149/http://www.gallaher.ie/
Cheers!