A 'representative peer' is a peer who, being a member of a class of peers lacking the automatic right to a seat in the British House of Lords, is elected by that class of peers to represent them in the House of Lords.
To date there have been three separate occasions when such elected peers have sat in the House of Lords as set out below.
Scottish Representative Peers
The Act of Union 1707 united the two kingdoms of Scotland and England and established a single parliament for Great Britain sitting at Westminster. At the time there were a number of Scottish peers who had previously attended the Scottish parliament (now abolished), and whilst their English cousins continued to have the right to attend the House of Lords, this privilege was denied to those title holders north of the border.
Some Scottish peers also held English titles, and were therefore entitled to a seat in the House of Lords by virtue of their English titles. The remaining Scottish peers who did not hold English titles (or subsequently gained titles in either the peerages of Great Britain or the United Kingdom) were, under the terms of the Act, obliged to elect 16 of their number to represent them in the House of Lords, for the duration of each parliament.
The remaining peers who were not so elected were not entitled to stand for the House of Commons.
These elections continued until 1963, by which time there were only a handful of truly Scottish peers left and under the Peerage Act 1963, the elections were abolished and all Scottish peers became entitled to sit in the House of Lords.
Irish Representative Peers
The Act of Union 1800 created the United Kingdom of Great Britain and Ireland and abolished the separate Irish Parliament. As before, in the case of Scotland there were a number of Irish peers who had previously enjoyed the privilege of a seat in the Irish Parliament, but did not automatically gain the same right to the House of Lords in Westminster.
A number of Irish Peers held titles in either the Peerage of England or Great Britain but those that did not were permitted to elect 28 of their number to represent them in the House of Lords. Restrictions were also placed on the creation of further titles in the Peerage of Ireland to ensure that their number did not exceed a 100.
Irish representative peers were, in contrast to the Scottish variety, elected for life, and elections conducted by the Irish Crown and Hanaper Office only took place when a vacancy arose. Those Irish Peers who were not selected were permitted to stand for the House of Commons and many did just that.
The last elections were held in 1919, after which time the Irish Free State came into being and abolished the Irish Crown and Hanaper Office in 1922, which made it difficult to conduct any further elections even if anyone had wanted to. The Irish representative peers continued to serve in the House of Lords until the last one died in 1961. In 1966 a group of Irish peers attempted to re-establish their right to representation but this was refused by the House of Lords.
British Representative Peers
The House of Lords Act 1999 abolished the rights of all hereditary peers to a seat in the House of Lords. However as a temporary measure, pending a final decision on the exact composition of a new 'reformed' House of Lords, all hereditary peers are entitled to elect 90 of their number to represent them in the House of Lords to sit alongside the Life Peers who now constitute the overwhelming majority of that body.
SOURCES
The 1911 Encyclopedia Brittanica entry for the PEERAGE see http://1911encyclopedia.org/index.htm
Charles Arnold Baker The Companion to British History (Longcross Press, 1996)