One of the best ways to sell your product, particularly to business customers, is through referrals and referral groups. A referral group is made up of members who have the same target customers but who are not in competition with each other. For examples, an accountant and a web developer targeting small & medium businesses in Dublin.
There are organisations such as BNI who run referral groups for a fee. You can also start your own referral group but it does require a lot of work. The biggest worry is allowing in a member who supplies a poor quality service or product. You have to be very careful who you let join.
The benefit for you is the cost is relatively cheap in comparison to traditional advertising or even online advertising. But it really depends on type of product or service you supply. In general terms, if the price is high or there is a significant cost resulting from purchasing a poor quality product, the more likely you will need to sell through referrals.