That's part of the reason. If it was the only reason they'd need to have the fund permanently locked, but they only lock when they think the fund is overpriced.
Let's say they valued their fund at 5B, but they start to see the market stalling. Maybe the feeling is property is 20% overvalued. (Example figures only)
They could (in theory at least) revalue that fund tomorrow at 4B and not lock and not need to sell properties - however that quick fall would cause investors to pull money out and the fund would eventually run out of cash and still need to lock.
So what they do is they quietly lock and over a period of a year or two gradually bring the fund valuation down by 20%. If they didn't lock they be effectively subsidizing client withdrawals until that fund price more closely matches reality.
They learned this the hard way in 2009(?) in their very first downturn when they delayed locking the fund and revaluing the fund, now they're trigger happy with the lock.
I personally think Irish Life should not make this fund accessible via pensions - it's not diverse, it can be locked for extended periods. They also in my opinion over invest in the fund via their more common default funds, i.e empower growth is ~7% invested in this fund.