Randalstown 2nd has a graveyard, so I’d start there. But people move around because of work or marriage etc and sometimes are buried in what may seem the strangest of places eg where their family lived 70 years before. People aren’t always buried in the graveyard of the church they attended. And many didn’t have gravestones.
For a Presbyterian around Randalstown there are graveyards in Randalstown OC, Randalstown 1st, Randalstown 2nd, Duneane, Craigmore, Drummaul Old (2 miles north of the town) plus some Presbyterians were buried in the Church of Ireland which for someone living in Derryhollagh might mean Randalstown or Duneane COI graveyards. Then there’s Cranfield, Shane’s Castle & Milltown graveyards too. But I’d start with Randalstown 2nd. Plenty of choice anyway.
Presbyterians generally don’t keep burial records, so if there’s no gravestone you will probably not find a record of where the grave is.
Given that Hamilton’s wife remarried, she may not be buried beside him. She’s more likely to be with her 2nd husband.