I am planning to fly from Delhi to Dublin-Ireland, transit will be at Istanbul for about 4 hrs.
Do I need transit visa for Istanbul?
If you don't leave the airport: no.
If you do leave the airport: yes.
You won't go through passport control in the airport if you are simply going from one flight to the next, so because of this you won't need a visa. But, if you have booked two seperate flights yourself, you may have to leave the transit area to collect and recheck your luggage, in this case you will need to apply for a visa.