Seatbelts?

Hi, my talbot has driver and passenger belts. Rear of mh has half moon seating around a table. Seats four sideways and two front facing. I have searched for legal guidelines on carrying passengers in the rear. Seems that if they are sitting sideways , they don't need belts, but do if facing the front. This Is uk law, couldn't find anything in irish law for campers?
My talbot is 1987,
Anyone advise me of the legal and insurance aspects of passengers in the rear.
Can I fit lapbelts to the forward facing seats and if I do, do these just need to pass the mot?
I'm sure I could weld a box section from chassis , through floor to mount the lapbelts too
Thanks

Try looking at Citizens advice for seat belts

Near the bottom is the exemptions

Exemptions from requirement to wear seatbelts
If you are sitting on a seat that is not fitted with a safety belt
If you are giving instruction in respect of the driving of a vehicle (i.e., you are teaching someone else how to drive). This is another example of a case where it might not be so wise to avail of the leniency of the law.
If you are driving test examiner conducting a driving test
If you are a member of An Garda Siochana or the Defence Forces and are driving as part of your duties
If you hold a medical certificate signed by a qualified medical practitioner stating it is inadvisable on medical grounds you wear a seatbelt or child restraint