Palatalized s/t in Irish

From: gprosti
Message: 65656
Date: 2010-01-15

I believe that Old Irish palatalized <s,t> (as in <secht>, <ticfaid> etc.) are pronounced in Modern Irish like English <sh> and <ch> respectively. Are there any theories as to when this change happened? Is it thought to have had anything to do with English influence?

Thanks