I would like to do the same comparison, as oil has got shockingly expensive in the last 2 years.
According the SEAI comparison, the cost of oil in cent/kWH is 8.29c and gas is approx 5c.
Oil and gas change price in response to international fuel prices at different times, so this is only a snapshot, but, it is probably good enough for the moment.
I have a 1800 sq ft 1950s solid block house with attic insulation. I currently use approx 1900litres of oil a year. At current prices this is €1600 a year.
If gas is approx 33% cheaper, this is a saving of €500 a year, which is pretty good.
If gas goes up and the price difference is only 25%, then the saving is €400, which gives us a range.
If the cost of putting in gas central heating is €6K, then the cost will be recouped in between 12 and 18 years. ( If anyone has a more accurate estimate for the cost of putting in gas heating, please let me know).
This is all very approximate, but probably accurate enough to give you an idea of the RoI.
You could try this comparisom using your own oil bills and see what it gives.