Redundancy- continuity of service UK to ROI

I moved to ROI in January 2005 when the company I worked for in the UK relocated its head office to Dublin. I had previously worked for the company in the UK for 17 years and I was made redundant from the Irish company in January 2011.
My severance package has been calculated on 23 years continuous service by the company I used to work for- no issue there.
However, I would like to claim 23 years continuous service and apply this to the Revenue SCSB tax allowance to significantly reduce the amount of tax on my severance package.
The Revenue are saying that I can only use the 6 years service in ROI and the 17 years in the UK do not count.
My contract of employment was fundamentally the same in the UK as it was in ROI- some minor differences in legal terms but nothing more.
The company in Ireland was a new company, created in mid 2004 but it did not start trading until Jan 2005. The company is a household name, with an American parent company.
I get the feeling that the Revenue don't really know how to deal with this and it appears that they have made a snap decision that only six years will count.
Does anybody know if their decision is correct, or how to challenge it?

The Revenue's own information suggests that years service in the State are only used in the calculation, so they may be following their own guidelines, and possibly even following the legislation to the letter of the law.

However, if you are being treated less favourably because of your residence and/or employment in another EU state than someone who was resident/working in Ireland, then this may be in breach of EU law and could be challenged.

You should first of all ask them to set out the policy or legislation that disqualifies your service in UK, and then set out your position to them, say that you are being treated less favourably and ask them to disapply any policy or legislation that is responsible for this treatment. If they refuse, then you would have to appeal the decision.

Thanks for your reply Ildanach.

I spoke to the revenue again today and was told that my UK service may be allowed and to instruct my previous employer to progress with the SCSB calculation and deduct tax using the 6 years of ROI service. I was told it may be possible to claim a refund later which is far from an ideal solution and I couldn't get a definitive answer regarding the policy they are working to.
I can find the SCSB calculation easily enough on the revenue website but I've drawn a blank looking for the policy that underpins it. Can you point me in the right direction to where I can find 'the letter of the law' please?
Also, when referring to EU law, is there a laymans guide that I can refer to? I just don't know where to start with backing up my argument that under EU law I am being treated unfairly.
Thanks in avance for your help, it is much appreciated.