No file return is required as DIRT is deducted at source by Irish/English banks. i.e. the interest your receive will already be net of DIRT tax.
If you look on the deposits best buys thread, it looks like the best rate for on demand deposits at the moment is 3.35% from Permenant TSB. Factoring in DIRT and no TRS, I make that a saving of (3.35%*0.75 - 2%) = 0.51% on €120k so about €615 a year.
This margin may increase or decrease over time so it might be worth checking every 6 motnhs or so. Once it drops below 0, it probably makes sense to paydown the mortgage.
I guess you need to decide if the savings are worth the hassle.
If you look on the deposits best buys thread, it looks like the best rate for on demand deposits at the moment is 3.35% from Permenant TSB. Factoring in DIRT and no TRS, I make that a saving of (3.35%*0.75 - 2%) = 0.51% on €120k so about €615 a year.
This margin may increase or decrease over time so it might be worth checking every 6 motnhs or so. Once it drops below 0, it probably makes sense to paydown the mortgage.
I guess you need to decide if the savings are worth the hassle.