I cold dyed some tots bots a lovely deep lilac and have plans to dye some more a 'kingfisher blue', my advice would be to make sure you 'stir' them regularly and limit it to 3 max per bucket (should be able to get 2 or 3 bucket loads but must be done at the same time otherwise dye doesn't take) whilst dying them and hot wash them several times before using them to make sure the dye is all out. I can send you pics if you want ;)
I haven't dyed any nappies, but have dyed plenty of other things (including pushchair seat & hood) very successfully with machine dye. It's really easy - no stirring, etc.! Just stick it all in the washing machine. Can't see why it wouldn't work for nappies too .
chipmonkey - depending on where in Ireland you are I am pretty sure you can get Dylon dye in Roches Stores in Cork for eg. Any department store with a decent haberdashery dept should do it - Arnotts in Dublin maybe?
the machine dye really is easy to use, and no problems with splodgeyness (technical term there ) which you can get with the hand dye if you don't stir enough or put too much stuff in to be dyed
haven't dyed nappies myself mind, but pretty much everything else under the sun has changed colour in my house, courtesy of Dylon