This could be 70's but more than lightly an 80's bottling.
If it is the 80's bottling it is probable that it was made up with the last of the remaining Gilbey's stock. This means that some much older whiskey has gone into the batch. That on the surface sounds great but it is actually the opposite ... some of the whiskey used spent way too long in wood and well past it's best. The end result is a disjointed slightly off whiskey.
If not it should be a little cracker so if Whiskeybro happens along he might be able to date this for you.
Strangely it will not overly have a major impact on the price. But the current state of finiancial affairs might ... the Irish whiskey market is not in a good place and prices have really fallen. I would hazzard a guess that if the bad one £80 if a good one £100.