IS stands for Islamic State, ISIS = "Islamic State of Iraq and Syria", ISIL = "Islamic State of Iraq and Levant".
Daesh is an acronym for the Arabic phrase al-Dawla al-Islamiya al-Iraq al-Sham (Islamic State of Iraq and al-Sham).
These are definitely synonyms, so they deserve a merge.
However, it is not that clear about what tag should be primary, simply because there is some kind of controversy about the term itself:
- Israeli politicians oppose calling Islamic State as "ISIL" because Levant is a historic landmark where Israel is located at;
- Some argue that: (markup mine)
Frankly, this evil death cult is neither a true representation of Islam, nor is it a state — UK Prime Minister David Cameron told Parliament in December 2015 when announcing that his government would be joining France in calling the group "Daesh" rather than "Isil". (BBC);
- Also, The root of the Isil-Isis inconsistency lies in the Arabic word "al-Sham". Al-Sham can be used to refer to "the Levant", "Greater Syria", "Syria" or even "Damascus". (source same as above)
My personal opinion is that Politics.SE should follow the official point of major governments and/or the UN, even if this would cause controversy among the users; such controversy seems to be inevitable.