This will answer some of your questions, hopefully other posters will add to the discussion.
Some of the disadvantages.
1. you have to charge VAT on the goods/services you sell, your goods will be more expensive to your customer.
2. you'll have to be organized and keep good records for your VAT returns
Some of the advantages
1. You can claim back the VAT you pay on your inputs.
2. It gives the impression to your customers that you are a substantial business, a cowboy trader will usually not be registered for VAT.
If your turnover is over a certain limit (about £60'000 in the UK, google it to find out what the limit is in Ireland), then you have no option but to register for VAT, it's a legal requirement.
Louis Cribben