Glens of Antrim, Strangford Lough, Mountains of Mourne, May/Sep are good months. Likely to be some events/sportives organised around the Giro but details of the actual route will not firm up until autumn,
There is usually a Sportive/club event every weekend from May onward with modest entry fees - £10-£25. e.g Dromara Coastal Challenge is on this weekend. Tour of South Armagh on 19th and Over the Quoile on the 26th May. All nice sociable rides with 50/100 people. Similar in June/July/Aug.