There are 166 TD's currently in Dail Eireann. There are 43 constituencies. A reduction of 1 TD per constituencies would leave 123 TD's in Dail Eireann.
Our Constitution demands that there be no fewer than 143 TD's at any time ( this is based on a min of 1 TD per 20,000 people and max 1 per 30,000), therefore a reduction of 43's TD's would probably mean a referendum.
We are actually very similar in TD/Population ratio as Bulgaria, Czech Rep, Denmark, Finland, Latvia, Hungary, Lithuania and Sweden.
As for money saved in a year, well the cost of holding a referendum would cancel out any savings made by reducing the numbers of TD's by 1 per constituency.