Parke's Castle is a little 1/2 drive east of Sligo. It's situated on a lake and there are some nice pull-off spots with pretty views along the way. The castle itself is kinda ho-hum. Nothing really fancy, but there is an escape route to the lake that's now overgrown with weeds. The west lawn is long and well shaded. You can walk through yourself or wait for a guide. The barn had an extensive display of horseshoes for some reason.
There is also a lake ferry that you can take. I believe it just goes out onto the lake and around an island and they explain what the island had been used for previously. I thought it was a waste of money, but surely could be a way to kill some hours if need be.