Cork is the largest county in Ireland and so you need to be more specific about location. Bantry House is very beautiful but is about an hour and 20 minutes drive from Cork City/Airport.
"If you fancy West Cork, the most popular wedding venues are Fernhill House in Clonakilty or Inchydoney Lodge and Spa, Clonakilty, on the seafront." Fernhill is very lovely but I always think of it as a venue for much larger weddings. Inchydoney is in a beautiful setting, food is excellent but it seems very modern, less romantic for a wedding venue. Dunmore House Hotel is in the same area, family run, been to several lovely weddings there in the big function room over the year, not sure if they have a more intimate space for a smaller wedding.
Will there be many guests travelling from outside of Ireland? If so, you could look at Cork city and Kinsale as both are linked to the airport by public transport.
Are the couple having a church wedding or a civil ceremony? Both partners must attend a face-to-face appointment with a Registrar at a HSE Civil Registration Service to declare intention to marry.
In the city, Hayfield Manor has an almost country manor feel about it and is intimate for a smaller wedding, lovely photo opportunities in the small gardens there and in nearby University College Cork. It's just a short distance (a couple of bus stops) to the city centre and there are plenty of B&B options on Western Road (downhill walk) for any guests needing less expensive accommodation. I've never been to a wedding there but we've had several family occasions to celebrate there and the food is always a delight.
For something a bit different you could consider Jacobs on the Mall, it's an excellent restaurant that can be fully booked out. It's right in the city centre on South Mall. The interior is very special, converted Turkish Baths. Slightly boho chic style. No outside space for photographs but there is a public park nearby. No accommodation on site but there are two hotels on the same street, The Imperial Hotel (also does weddings) and The Maldron.
On the outskirts of the city are Maryborough Hotel mentioned by a previous PP, Vienna Woods Hotel and the Kingsley Hotel. We were in Vienna Woods Hotel lately for a family gathering after a funeral, meal was delicious, thought the room would be lovely for a smaller wedding but they do have a much bigger function room with dance floor.