Some more information here:
https://www.eggsa.org/1820-settlersArticle "The Story of the 1820 Settlers"
Link to a book on the Internet Archive "British South Africa - A History of the Colony of Cape of Good Hope from the Conquest 1795to Settlement of Albany by British Emigrants 1819" by Colin T. Campbell, published 1897. A short chapter on Parker's Party. List of emigrants by ship and party further on in book. Alexander, Edward and Ed's wife were all recorded as aged 27. (The writer of the above article mentions age discrepancies for some settlers.)
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1820 Settler places in British Isles - select Ireland then Longford and there's a pic of Templemichael Church (St. John's C. of I., Church St., Longford Town). John Armstrong (another settler) married Catherine Forbes there.
Additional information - Surnames - F - Forbes. Says Edward's age at death in 1870 was "about 100" which was not correct. Suggests Catherine Forbes, wife of John Armstrong was probably sister of Edward and Alexander. Mentions baptism of John, son of Alexander Forbes and Elizabeth Clarke, January 1817 at Templemichael.
Correspondence from or about settlers. Collection of correspondence is in National Archives at Kew. Some transcriptions are in Cape Archives CO 6138. There are some transcriptions online at wa.amu.edu.pl/1820settlers_petition_letters/start
Correspondence includes 2 letters written from Cobh, Cork by some settlers from Longford while they were waiting to find out if they could sail and becoming desperate. One is by John Armstrong, possible brother-in-law of Alexander and Edward Forbes and a follow-up letter a couple of weeks later from Ed. Forbes and John and Laurence Armstrong "after selling our small places …".
Sorry if there are any mistakes above. I can't go back and correct them or I'll lose it.
Ang may know all the above.
Alexander was killed in S. Africa.