We focus on the prices of condominiums traded in the Greater Tokyo Area from September 2005 to December 2009. According to the register information published by the Legal Affairs Bureau, the total number of transactions for condominiums carried out in the Greater Tokyo Area during this period was 360,243. Ideally, we would like to have price information for this entire “universe,” but all we can observe is part of this universe. Specifically, we have three different datasets, each of which is sampled from this universe.

This dataset contains initial asking prices (i.e., the asking prices initially set by sellers), denoted by P1, and final asking prices (i.e., asking prices immediately before they were removed from the magazine because potential buyers had made an offer), denoted by P2. The number of observations for P1 and P2 is 155,347, meaning that this dataset covers 43 per cent of the universe.

Real Estate Information Network System, or REINS, a data network that was developed using multiple listing services in the United States and Canada as a model. This dataset contains transaction prices at the time when the actual sales contracts are made, after the approval of any mortgages. They are denoted by P3.

The third dataset is compiled by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). We refer to this price as P4. In Japan, each transaction must be registered with the Legal Affairs.

Ok, in light of jxbr's helpful comments on what figures I should and shouldn't include, I'm updating the calculation.

You can start to surmise property sales volume based on the IBF figures for mortgages drawdowns

(Note 1: I exclude Top-ups, as IBF defines it as "a further mortgage advance to an
existing borrower which is issued to finance expenditure other than house purchase."

Note 2: I've included 50% of each quarter's Re-Mortgages figure, as IBF defines it as "a loan which is issued by one lender to refinance an existing mortgage with another lender. This may or may not include further equity release.")

Q1, 2011 2,454
Q2, 2011 2,863
Q3, 2011 3,055
Q4, 2011 3,326
Q1, 2012 2,274
Total 13,972

Therefore, average monthly sales volume from Q1 2011 to Q1 2012 is 931 (13,972/15)

IBF states "We estimate that the data covers well in excess of 95% of the mortgage market."

So let's add 5%, making it 978.

Now the CSO's coverage of stamp duty returns in 2009 (laughably the latest figures at hand) showed cash sales of 6%. Numerous estate agents (with vested interests) have been saying cash sales are now 25-40% of all sales.

So let's say the 978 mortgage drawdowns per month on average from Q1 2011 to Q1 2012 represents 70% of all property sales in the period.

We can add 30% for cash sales, making total monthly property sales 1,271.

Ok, a few assumptions gone in, but interesting to see there currently may be as little as 1,271 transactions per month nationally.

And this is between all 8 types the CSO is tracking:
1. National - all residential properties;
2. National - houses;
3. National - apartments;

4. National excluding Dublin - all residential properties;
5. National excluding Dublin - houses;

6. Dublin - all residential properties;
7. Dublin - houses;
8. Dublin - apartments

Now, the 2010 Stamp Duty figures were presented lately in the Dáil
http://debates.oireachtas.ie/dail/2011/12/13/00049.asp

(Sorry for comparing 2011 mortgage drawdowns with 2010 SD returns. Just can't get the 2011 SD returns yet!)

It shows 43% of the receipts were for Dublin.

So using this, of the Q1 2011 to Q1 2012 average showing 1,271 property transactions per month in Ireland,
547 were in Dublin,
724 were in Rest of Ireland.

I’ve checked accuracy of my estimates above versus the official PPR figures.

From Q1 2011 to Q1 2012 Actual Property Transaction Volumes
[All 22,141, of which Dublin 7,107 (32%) and ROI 15,034 (68%)]

I had average monthly volumes for

All 1,271. Per PPR 1,476 (22,141/15). [So out by = -14%]
of which Dublin 547. Per PPR 474 (7,107/15) [So out by = +15%]
of Rest of Ireland 724. Per PPR 1,002 (15,034/15) [So out by = -28%]

Interesting to see that Dublin in 2010 represented 43% of Stamp Duty returns by Value but 33% by Volume.

I don't know if this was intentional, but when you added 30% in your original post, were you thinking
"OK, 30% are cash sales, so I'll add 30% to that figure (978)"?
Because if you were thinking that, there is a small error in your calculation.
What I mean is, 978 was 70% of the market, so divide by 7 and multiply by ten to get 100%. For simplicity, let's round to 980 instead of 978.
980/7 = 140
140 x 10 = 1400, getting you quite a bit closer to the real figure.

Average monthly sales volume from Q1 2011 to Q1 2012 is 931 (13,972/15)

IBF states "We estimate that the data covers well in excess of 95% of the mortgage market."

So let's add 5%, making it 980 [(931/95)*100].

Now the CSO's coverage of stamp duty returns in 2009 (laughably the latest figures at hand) showed cash sales of 6%. Numerous estate agents (with vested interests) have been saying cash sales are now 25-40% of all sales.

So let's say the 980 mortgage drawdowns per month on average from Q1 2011 to Q1 2012 represents 70% of all property sales in the period.

We can add 30% for cash sales, making total monthly property sales 1,400 [(980/70)*100].