Ilha can easily be reached by road from Nampula. If you're driving follow the road towards Nacala, and turn off at the signpost for Ilha de Mozambique which is in Monapo. It'll take around 2 hours and the road is good. When you get there you'll need to cross the causeway which costs about Mts 5,000.
If you're taking a chapa they leave from Ave de Trabhadores in Nampula near the railway station and cost Mts 80,000.