Oaths of Britain
This is a fantastical alternate history, so we are going to assume that post is affordable and reliable, though it will still be slow. Based on some calculations here, we can estimate that a letter can travel approximately 50 miles per day. Sending post to Ireland adds 1-2 weeks, as arranging for a vessel across the Irish Sea and then dealing with odd Irish political entities are both inevitable complications.
As mentioned elsewhere, all correspondence is assumed to be ICly in Latin. OOCly, everyone should be writing in English so that our final compilation is readable by everyone. If you would like to write IC in another language, please indicate on the top left-hand corner of your letter what language you’re writing in, like so:
L: Norman French
You can also use this format to switch languages within a letter. Just use brackets or something to indicate where the language begins and ends.
The basic format of a letter hasn’t changed significantly from Roman times to the present, so any formal letter style will work as an example.
The SCA has some helpful links on medieval letter-writing (sorry the links are borked!)
http://www.scatoday.net/node/2491 – Writing the Medieval Missive
http://www.dragonbear.com/letters.html – Aids to Letter-Writing
http://wac.colostate.edu/books/textual_dynamics/chapter4.pdf The Medieval Art of Letter Writing (PDF) – academic book chapter on medieval letter-writing. Helpful, if dense.