There are a few important background and technical matters to bear in mind when interpreting this ICJ opinion.
First it is an opinion, that is to say the legal opinion of the ICJ judges. It is not a judgment (if by judgment people think of a determination of guilt or innocence).
Second, it is a non-binding opinion. In other words there is no obligation for other parties to change their practice to agree with this opinion. (For example, state don't have to rush out and recognise Kosovo). What the ICJ tries to do, it claims, is that “the Court’s advisory opinion would provide politically neutral, yet judicially authoritative, guidance to many countries still deliberating how to approach unilateral declarations of independence in line with international law."
Third, an opinion of the ICJ was requested by the UN General Assembly, not by either the provisional government of Kosovo or the state of Serbia. The specific question that the UNGA requested an opinion on was ‘Is the unilateral declaration of independence by the Provisional Institutions of Self-Government of Kosovo in accordance with international law?’”
The main purpose of the advisory opinion is the purpose of the advisory jurisdiction is "to enable organs of the United Nations and other authorized bodies to obtain opinions from the Court which will assist them in the future exercise of their functions. The Court cannot determine what steps the General Assembly may wish to take after receiving the Court’s opinion or what effect that opinion may have in relation to those steps."
Fourth, what is quite interesting (at least to me ) is that written submissions to the Court came from 35 state parties (+ the Kosovan provisional institutions), including from Cyprus but not from Turkey. This is surprising in one respect because one would have thought that Turkey would take the opportunity to argue the TRNC case.
Remember that the ICJ has not given an opinion on Cyprus/TRNC because neither state parties (most obviously RoC and/or RoT) nor UN agencies/organs have ever requested the ICJ for an opinion.
Addressing this thread's question "What effect does this have on the Cyprus problem?", some of the usual suspects have already denied that it has any relevance to the Cyprus question. That's not surprising since actually this opinion has a
direct implication for the Cyprus question although it will not change things over night. As an opinion what this does is simply add a little bit more weight to the expression of the right to self-determination. Most importantly it makes the key distinctions as follows :
"it asks for the Court’s opinion on whether or not the declaration of independence is in accordance with international law. It does not ask about the legal
consequences of that declaration. In particular, it does not ask whether or not Kosovo has achieved statehood. Nor does it ask about the validity or legal effects of the recognition of Kosovo by those States which have recognized it as an independent State."
The Court had to wrestle with the submission of some parties that
"international law does not regulate the act of making a declaration of independence, which should be regarded as a political act; only domestic constitutional law governs the act of making such a declaration, while the Court’s jurisdiction to give an advisory opinion is confined to questions of international law."
In the end the Court argued that
"The General Assembly has requested the Court’s opinion only on whether or not the declaration of independence is in accordance with international law. Debates regarding the extent of the right of self-determination and the existence of any right of “remedial secession”, however, concern the right to separate from a State."
Applying this reasoning to Cyprus, the ICJ by analogue is saying that it is not for them to adjudicate or give opinion on whether TRNCs declaration of independence from RoC is illegal or not (that is a matter for RoC domestic law). But when it comes to international law, over which ICJ does have jurisidiction, it is saying that declaring independence is
not illegal under international law. That is to say, such declaration do not breach any international law :
"the Court need only determine whether the declaration of independence violated either general international law or the lex specialis"
and it concluded that the ICJ
"Is of the opinion that the declaration of independence of Kosovo adopted on 17 February 2008 did not violate international law."
In keeping with ratio of this ICJ opinion, I believe therefore that had an advisory opinion of the ICJ been requested regarding TRNCs declaration of independence the opinion would have been the same : "it did not violate international law."