You don't have to be a qualified plumber to become a gas installer. FAS run courses but you will need a relevent qualifacation, refridgeration, air con, plumbing etc. if you don't have these you will need a letter from your employer and submit referances to show you have experiance in the industry.
The courses won't teach you how to be an installer, they go through safety and procedure, along with Irish legislation. it is very important you get hands on experiance with a competent person, this takes a couple of years. Even if you were to get the courses done, to become a registered installer you must submit to Bord gais, six recent jobs carried out by you and then they send out an inspecter. this along with valid insurance, current H&S statement, owning copies of appropriate current legislation, company profile and more!
Many boiler companies run free servicing days but you need to be a qualified gas engineer to avail of them.