Yes, bus as well as train is a good idea in Ireland, to avoid a lot of doubling back and getting to places not served by rail.
Northern Ireland is simpler to plan as one company, Translink, runs almost all internal public transport. Something to keep in mind is that a day ticket (issued on a locally available smartcard) is £19 for all train and bus in NI so may be a better option than interrail.
https://www.translink.co.uk/
Here is a bus and train map, be aware that many routes are very low frequency.
https://trn-prd-cdn-01.azureedge.net/mediacontainer/medialibraries/translink/route-maps/ulsterbus/ulsterbus-route-map.pdf
ROI is a little more disjointed as buses are run by multiple companies and there is no single ticketing system.
Map here, again frequency varies widely. Another thing to consider is traffic, do not rely on buses being punctual.
https://www.transportforireland.ie/wp-content/uploads/2025/10/Regional-Public-Transport-Map-2025-August02.pdf
The TFI journey planner is useable for point to point planning. https://www.transportforireland.ie/plan-a-journey/
Some scenic highlights worth mentioning:
Derry - Coleraine railway.
Belfast - Larne railway is also quite scenic but Larne isn’t somewhere worth any time itself.
Coleraine - Ballycastle “Causeway coaster” 172 bus, runs every 30 minutes.
Summer only (July and August) 252 bus Larne - Ballycastle, very scenic route along Antrim coast, only runs 2 times a day.
The most scenic line in ROI is Dublin - Wexford - Rosslare, to avoid doubling back there are bus routes towards Waterford, also Waterford - Cork.
There are numerous scenic routes, mostly off the railway network.
Connecting the ends of Rail lines in the west is not a problem, Galway to Ballina , Westport, Sligo and inbetween these is very possible, also as said Sligo to Derry via Co Donegal.
Co Donegal itself has spectacular scenery but is very rural and no railways, possible by bus but would take some time and good planning.