1. You have to buy an annual policy, unless you are involved in work with a high exposure, you should be looking at a reasonable minimum premium €750 to €1,000.
2. There is no advantage to dealing directly in the UK. Firstly the market at present is at its lowest level of pricing in five years. Maybe not as low as general UK prices, UK underwriters charge rates for Insurance in Ireland based on the environment here ie claims experience mainly, they are not going to charge UK rates and pay out Irish level of claims.
It is possible to get competitive arrangements from your local commercial broker in the present market. Hope this helps.