Using "t" for "th" in many words is actually quite common in England from people who aren't trying to sound Irish. And an alternative favoured pronunciation for "th" is more akin to "f".

There's a lot of variation in how people speak, and often enough we just don't notice.

Joybell said "there was a change in pronunciation of many words when more people became literate" - and that seems to be true enough - except that I'd be inclined to say "semi-literate" to describe the assumption that written English is a lot more regular and phonetic than it ever has been.