I don't think there is a regulation around this because my gas boiler is in my kitchen! although it may have been placed in there many years ago. You could phone the RGII for info?
A room sealed boiler, may be located within any room of the dwelling. If located in bathroom / shower, in an enclosed compartment or understairs, additional requirements must be adhered to. (I.S.813: 2002). Additional requirements are needed if placing a boiler beneath stairs.
My gas boiler in kitchen too. However I was watching an item in Tv3 re carbon monoxide poisoning and they had the actor from Mrs Brown's Boys on. Rory? Anyway he had his house remodelled and boiler upstairs and became quite sick. Advice was to always have downstairs.
Wherever you put your gas boiler remember there will be emissions outside from it when in use (Duh!). But, how close is your gas emission pipe to your next door neighbour's side entrance? And if you can walk by the side of your house will you are your kids be inhaling the fumes? - Oh! yes if your boiler is to the back of the house, how far from backdoor or back windows is the emission pipe? Just some thoughts.