In my little green book entitled Collins Pocket Irish Dictionary, there's a section on letter writing at the end, with sample letters and beginnings and endings. In the list of 'less formal' it has -
Goodbye - Beannacht
Bye for now - Slàn go foill (with a fada on the 'o')
All the best - Adh mor (with fadas on the 'A' and 'o')
Much love - Grà mor (with a fada on the 'o')
Your friend/pal, - Do chara,
With love from - Le grà o.... (with a fada on the 'o')