"should my son and I be joint owners or owners in common of the Irish property we buy together? "
For tax, it doesn't matter - any amount gifted/inherited to your son above the CAT threshold is taxable- whether you hold as Joint Tenants or Tenants in Common.
I'm sure there are very good reasons why you're selling a house, to gift money to your son, so that he promptly buys a property with you, with that money, and an additional 35K but it does seem excessively complicated if the intention is purely tax driven?
mf