According to their website, there are no direct flights from Belfast International Airport, to Belgium:
http://www.belfastairport.com/en/content.asp?a rea=2&id=210
Similarly, there are no direct flights from George Best City Airport:
http://www.belfastcityairport.com/template1.as p?pid=39
Ryanair fly from Dublin to Charleroi Airport (which is just outside Brussels).
http://www.ryanair.com/site/EN/
I've just checked a few dates and there are plenty of flights at �3.99 each way. With taxes and charges, that means a return flight will cost you �45.51.
The rail desk at Charleroi airport will sell you a 'bulk ticket' for just �10.50. That includes the 10 minute bus ride to Charleroi Sud rail station and then onbound rail travel to any station in Belgium. (For many destinations, that means taking a 45 minute train journey to Brussels Zuid and a connection from there).
So, excluding your travel to Dublin airport, you should be able to get anywhere in Belgium for under �70 return.
Chris
(PS: Your post mentions travel by ferry. Don't even think about it. There are no direct services from Ireland. You'd have to take a ferry to England (or Wales) and then a lengthy, and very expensive, rail journey to Ramsgate, followed by a ferry to Ostend. Stick with flying ; it's far, far cheaper and much, much quicker)