A thought occurs to me halfway through the Rhythm Ride spin class at Perpetua Fitness: have I ever sweated as much as this? For 45 minutes, in this subterranean corner of Dublin’s Portobello, I pedal furiously, mostly off the saddle. The idea is to cycle to the rhythm of the music — hip-hop, power pop and Whitney Houston — and to experience a workout that’s more about fun than hardship.