I struggled with names ending in -cester when I first got here. For weeks we listened to radio announcers talk about some place called "Toaster" and another place called "Bister". Finally I bestirred myself to look at a map and came up with "Towcester" and "Bicester". Aha, the penny dropped. Where -cester is preceded by one syllable, you get Towcester = Toaster and Bicester = Bister, and likewise Worcester, Leicester and the rest. Where there are two syllables preceding the -cester, such as in Cirencester, the -cester is pronounced more or less as spelled = "Cyrensester".

Irchester, on the other hand, has the "h" which guarantees a full pronunciation of the last two syllables regardless of what precedes it.