Hi,
Without any position updating, your IRS position will drift within a tolerance published in the manuals somewhere. On our TriStars the INS error was (3 + 3T) nms where T = (Flight Time + Taxi Time) in hours.
On the Airbus with IRS, we simply look at the residual ground speed error when parked, and do a rapid re-align if it's less than 5 kts or a full re-align if it's greater than 15 kts. (I don't have the info to hand for maximum allowable drift error.)
The FMC adds a position bias (calculated from DME/DME fix or GPS) to the IRS position to produce your MAP. Provided your GPS position was good - you would still be able to do RNP 1.
20 miles error in 8 hours would have been well within our old INS tolerance.