If you check online for the best places to see fireflies in Taipei, then one of the locations that appears in many of the lists is Dagouxi Waterfront Park. In fact, this is a little misleading since - while the walk does start there - you need to head a little further into the dark to spot any fireflies.
where to see fireflies in taipei
FIREFLIES at CUEI LAKE (汐止翠湖螢火蟲)
Cuei Lake in Xizhi District is one of several firefly spotting locations in New Taipei City. Visit in April to enjoy this beautiful annual event.
TUCHENG TUNG BLOSSOM TRAIL (土城油桐花步道)
Every April and May the mountains around Taipei are cloaked with the white blossom of the tung tree. Getting out to observe this May 'snowfall' is a popular activity amongst the locals, and this trail in Tucheng is one of many places you can go to join in.
FIREFLY SPOTTING IN TAIPEI ON TIGER MOUNTAIN (虎山的螢火蟲)
You don't need to travel to the remote mountains of central Taiwan to experience the magic of seeing fireflies lighting up the evening forest. Hushan has a trail within walking distance of the MRT which is adapted for firefly viewing in peak season. If you visit Taiwan in April or May, you should definitely set aside an evening to step into this enchanting realm of fireflies, night birds and noisy frogs.
Taiwan Trails and Tales 