English has a phenomenon known as linking R. This only occurs in non-rhotic varieties of English They pronounce the /r/ when it is in the syllable onset and occurs before a vowel. In the case of linking R, however, the /r/ is word-final.
Non-rhotic varieties of English pronounce word-final /r/ when the following vowel begins with a vowel sound and there is no pause between the segments. For example, the /r/ of winner can be pronounced in the phrase the winner is. However, in the English of many non-rhotic speakers of the southern USA, linking R fails to apply.
Non-rhotic varieties of English often apply linking R. This /r/ always occurs across word boundaries. It isn't categorical, however, because it's blocked if there's a pause between the two segments.