We were told by our mortgage advisor that once a letter is given by the person giving the gift confirming that it is indeed a gift that will satisfy the bank.
The level of gift you can recieve (from a Revenue point of view) is dependant on who is gifting to you. Revenue have three different thresholds for different types of relationships (e.g. Type A is a gift from a parent to a child, Type B is from a child to a parent or between syblings etc.).
Lots of discussion of the CAT thresholds on AAM and full information available from the Revenue website (www.revenue.ie) [Some relevant information [broken link removed], but slightly outdated].
I'm not sure I see the relevance of your points 1 and 2.
1. If the person "gifting" you the money has a legal stake in the house, this is not a gift. It's an investment. This will have huge implications on the purchase as the bank has to take account of thier legal ownership.
2. If the gift is a loan, again, it's not a gift. Loans will be/are treated differently so it's a very different situation (One I'm not familiar with so can't comment on it tbh).