According to the English Language Wikipedia page on Cirencester, Cirencester has many pronunciations. From Wikipedia:
The form /ˈsɪsɪtər/, spelled Cirencester or Ciceter, was once used locally. This pronunciation is humorously highlighted in a 1928 limerick from Punch:
There was a young lady of Cirencester Whose fiancé went down to virencester By the great Western line, Which he swore was divine, And he couldn't have been much explirencester.
I don't understand the humour in this limerick. I think the words virencester and explirencester do not exist. But are they variations of existing words? Where is the humour in this?