I don't have a lot of knowledge about this, but it seems that McGuinness resigned as a protest over the handling of a renewable energy project. His resignation is likely to end the power sharing arrangement and trigger a new round of elections. Since Northern Ireland has a first minister and deputy first minister with equal power, if one party resigns, the government may fall.
Northern Ireland has had a great deal of problems lately and the energy scheme disagreement was the proverbial straw that broke the camel's back. This disagreement is at the core of many of Northern Ireland's problems. It is also important to remember that there are years of history that have divided Ireland.
Although an issue such as the renewable energy scheme in Northern Ireland may cause tension, it could not possibly be solely responsible for a government collapse. Whenever there is failure in the government or any other such organization, it is likely compiled from several different areas of decline, corruption, or failure. Although this may contribute, and be a big part of a government collapse, it could not be the sole reason. There are always underlying issues.
The North Ireland government has been on shaky ground for years due to lack of strong leadership. I hope that this crisis leads to new discussions on reuniting the island under one government. The Irish do not have enough say in their own laws and politics and that is a terrible shame.