If you've read Jack Higgins before (in particular anything involving Sean Dillon) this book will turn up no surprises... if follows the adventures of Dillon (ex IRA, then mercenary, now working for Brigadier Charles Ferguson in the depths of the British government).
This time Dillon's past catches up with him... 10 years earlier he infiltrates a Loyalist planned bullion heist but it does not go to plan, and the bullion sinks to the depths of the Irish Sea. Now however, word reaches Dillon that there is a plan to salvage the bullion, and Dillon needs to save the day (having firstly shocked everyone by confirming he was the mystery accomplice on the original heist).
Great literature it is not, but to be honest as Dillon books go, this one isn't bad... Liam Devlin (Dillon's mentor who sometimes makes an appearance) does indeed make an appearance here and the Salters (Harry and Billy) are introduced for the first time. If you have read Jack Higgins' Dillon books before and enjoyed them, then I'd recommend this one; if you've not read Dillon books try out Thunder Point or the one before it (as that introduces you to the main characters). Ah, and if you're not a fan of Dillon, then you'll already have probably decided not to bother.
Expect lots of Bushmills, some pub grub, scuba diving and Dillon taking on ten armed men and winning... all in a day's work really.