first lady peer 1871

[e]

who was the first lady peer in 1871?

[old jawn]

This would seem to be a reference to Angela Burdett-Coutts, who was left a half-share in Coutts Bank in 1837 and became a social reformer and philanthropist, as well as head of the bank, and was made a peer in 1871.

Royal mistresses down the years would have beaten her to the peerage, but, apart from that category, Baroness Burdett-Coutts could be argued to be the first woman in the peerage, or at least one of the first.

[Aquinas]

There was also the Countesses of Mar, a Scotish title that can pass to females.