In the North, commercial diving is regulated by the HSE. I believe Eire has its own Safety body which uses the U.K. HSE Diving regs as a guide.
If you are setting yourself up as a shellfish diving company, you have to have one of these qualifications:
http://www.hse.gov.uk/diving/qualifications/approved.htm#sch3
And here’s a link to the HSE shellfish regs:
http://www.hse.gov.uk/hid/osd/scallop.pdf
In addition to the above, you would have to have a fisheries quota, and pay tax on your income.
Alternatively, you could just look for a berth on a Scallop diving boat. Then you would just need the diving qualification (I believe there’s a CMAS or PADI ticket which would allow you to work) a medical and First aid ticket. You would earn whatever you catch minus a cut for the ‘boat’ and leave the skipper/owner to deal with selling and the business side of things. Bear in mind that you only earn on your own catch, so if you are **** at it, you will go hungry (and if you’re really ****, the skipper will want someone else to fill your slot so he can make money too).
There are a few companies based around the west coast of Scotland and the isles, with a few ‘hot spots’ like Mallaig or Orkney- I’m sure a bit of google will get you some more info (I’ve seen ads for divers on ‘ukdiving.co.uk’ and ‘findafishingboat.co.uk’ for example.
Or you could just say ‘****it’ and just go diving for scallops on your own. Many hotels would buy off you cash in hand, but you would be breaking the law, the Tax man would want to talk to you, the Fisheries Protection guys would want a word too- not to mention the Scallopers who are working where you go (often the purveyors of swift ‘justice’ if they have to pay for quotas, tax, MCA compliant boats and find you muscling in on their market...)
It’s nice to go for a ‘fry’ in the summer evenings, but I have seen a lot of guys who start seeing pound signs on each scallop and ‘dive in’, not focusing on how precarious an existence it may be (one guy I know jacked in his diving job, bought a license, RIB and a pickup, a compressor and a pile of scuba gear with a bank loan and made a fortune in Shetland.....for around 4 months, when the weather closed in for the winter. Then a few months of not finding any decent beds and he was bankrupt).
Also, if you go for it, don’t push it too much. The Old divers do 2 dives a day and earn a little, the Bold Divers do 4 dives a day with stops and earn a lot..until they end up in the Chamber!