Upvote:2
As stated in Timatic, the database used by airlines:
Visa required, except for holders of normal passports issued to nationals of Ireland (Rep.) can obtain a visa or an e-visa on arrival, for a max. stay of 3 months. The e-visa can also be obtained online prior to departure at www.evisa.gov.tr.
In other words, you can either apply through the website, or through kiosks at Istanbul airport.
Online it costs USD 20, while at the airport it costs USD 30 (or EUR 25, or GBP 20)