It is understood that this question refers to enabling people who work in the fishing sector to work in the offshore development sector. On this basis the reference in the question to “offshore fishing" is taken as meaning the “offshore development" sector, which might include offshore renewable energy developments.
In this regard, I wish to advise that the requirements for certification as an Able Seafarer are set out in the European Union (Training, Certification and Watchkeeping for Seafarers) Regulations 2014 and the Navigation Examination Directions of the Department of Transport. There is no legislation in Standards of Training, Certification and Watchkeeping for Seafarers (STCW) prohibiting persons from being employed onboard mercantile vessels as uncertified rating seafarers, provided that they have completed the four elements of STCW basic training and are not forming part of a navigational watch on a ship of 500 Gross Tonnage (GT) or more, other than as rating seafarers under training, and rating seafarers whose duties while on watch are of an unskilled nature.
Information on Maritime Education and Training in Ireland is available on the Department's website: https://www.seafarers.ie/
Information on the certification of rating seafarers is published in Chapter 9 of the Directions as to the Examination of Deck Officers and Ratings under the Merchant Shipping Acts (The Exam Directions), also available on www.seafarers.ie.