A study published in the International Journal of Global Energy Issues concluded that a solar thermal system installed on the Deshmukh Agricultural University campus in Maharashtra state, India had a payback time of just two years, and would effectively pay for itself five times over given an estimated working life of 20 years. Obviously India's climate is a bit different to Ireland's.
Another study has concluded that semiconducting meterials less costly and just as abundant as silicon could substantially reduce the cost of solar PV. The research group examined 23 promising semiconducting materials, and concluded that twelve are abundant enough to meet or exceed global demand. Of those twelve, nine are significantly cheaper than silicon.
ScienceDaily also reports that the cost of photovoltaic systems declined significantly in the US between 1998 and 2007.
