Corry's been shown in Ireland since 1978.

http://en.wikipedia.org/wiki/Coronation_Street#International...

Not saying it's not the reason for the divergence, but it seems less likely.

A lot of Irish households have access to British TV stations like BBC, ITV etc. and could have watched it there.

Since Deirdre is an Irish name, it is possible that the decline in popularity (among the general population) in the UK was at least partly due to increased anti-Irish sentiment in the 1970s.