Irshalgad can be reached from either Panvel or from Karjat. You need to reach Chowk which is on Panvel-Karjat highway. You may take a bus or auto from either Panvel or Karjat to reach Chowk.
From chowk it is 2 hour trek to reach the pinnacle of Irshalgad. There is also a water reservoir on the way. The trek is very easy and has may view points on the way. During monsoon, the area is filled with greenery and covered with clouds.
Make sure you carry sufficient amount to water and food before you begin the trek as you will not find anything once you begin the trek.