Just be careful over-estimating the money back.
1 - they will need to work out the correct or most apt margin. This will probably be the highest of the margins they had generally available. - Possibly as high as 2% and unlikely to be under 1.5%
2 - Interest rates have been very low for years (thankfully), so interest on the overpaid interest will be minimal
So assuming all goes as everyone hopes, then, as a ballpark, I would estimate that for each €100,000 of mortgage balance at Jan 2010, the refund would be about €12,000 - €13,000.
Or even easier, 12%-13% of the mortgage balance on Jan 1st 2010 add about 2,000 - 2,500 for compensation and time value money.
So ballpark, 14% - 15% of what your mortgage balance was in Jan 2010 would be the figure I'd put in my head and graduated lower by about 1.2% a year for those whose fixed rates ended in subsequent years. (2012 = circa 12% of balance etc)
This is pure guesswork based on amoritisation tables and purposely on the lowish side, using 3.75% as average svr and 2% as the tracker margin - I was fairly accurate with my estimations on KBC and Ulster, so this won't be far off. (I hope)