Northern Ireland is not the best climatic area within the UK to grow vegetables. Average temperatures are lower than most of England and Wales, the risk of frost continues longer and starts earlier and then there is the rain.
If I could warrant the expense I would have a large heated greenhouse that would allow me to have a much longer growing season. That said my unheated version still helps and for several years I have used a small heated propogator to get seeds germinating earlier and quicker than the greenhouse permits. The problem was that it was just too small. Accordingly I have now invested in a much larger propogator which will allow me to have several seed trays on the go at the same time. There is a thermostat incorporated in it so I am able to regulate the temperature with some degree of accuracy. The propogator was made by a company called Greenhouse Sensation. They have christened their product, "the Vitopod."