The principle cause of the Great Famine was Phytophthora infestans. This so-called 'blight' (scientific understanding of it at the time was very dubious) thrived in Ireland in the mid 1840s partly due to favourable climatic conditions (cold, wet), and the virtually exclusive use of a strain of potato particularly susceptible to the infestation.
On the eve of the famine, the potato constituted approximately 60% of Irish food requirements (according to P.M.A Bourke) - the blight, over the course of three seasons, effectively destroyed this vital crop. A common claim, by the contemporary nationalist John Mitchel, among others, is that Ireland produced enough food (principally in grains) to overcome this severe deficit in calorific intake - this is the cornerstone premise of any accusation of 'artificial famine'. But according to Peter Solar, exports after 1845 (when they dropped to around a third the value of preceding years) and after the devastation of the potato amounted to 15% of total daily calories (or 1.9 billion). So, firstly: the shortfall in calories due to the blight was too large to be plugged by domestic product being exported; and secondly, after 1846, emergency imports of grain exceeded exports by a factor of nearly 3:1 (or 5.5 billion calories).
Concisely: Mitchel's accusation of an 'artificial' cause for the famine is completely unsustainable upon an examination of the statistical data, and subsequently any claim of 'genocide' verges on the utterly absurd. Sorry other poster who made this claim - you are wrong.
That said, Mitchel's criticisms (insofar as they appropriately characterize the position of radical Irish nationalists on the topic) of the relief measures enacted by the British contain at least an aspect of the truth: 'infatuation' with laissez-faire economic policy; anti-Irish prejudice, and religious dogma severely hampered prompt and effective famine relief. The 'providential' view saw the famine as God's will, a kind of punishment for Irish popery; the 'moralistic' view identified deficiencies in the character of the Irish people as a whole for instigating the disaster: the peasants being idle beggars, the landlords corrupt parasites. These ideological motives informed the uneven and often ill-mannered British response, from the workhouses to the evictions. I think it's entirely fair to say it could have been done better. But, if we accept that the famine was indeed a natural disaster outside of British control to forestall, then its miserable efforts to relieve it must surely fall short of actual murder?
I'd recommend James S. Donnelly's The Great Irish Potato Famine for an accessible and impartial historical work on the topic, and John Mitchel's Last Conquest of Ireland: Perhaps as the most influential articulation of the nationalist indictment of the Union - one might detect a certain degree of hyperbole in Mitchel's claims of 'slaughter', but his venomous wit and invigorating prose is well worth a read. It's a complicated topic, with too many facets to illuminate in brief.
I studied and researched the potato famine from a comparative perspective. I also helped out with a friend who wrote his thesis on the potato blight in Belgium. I've answered this question once or twice before, but I can't find them so I'll simply add to what has already been said.
The lack of a comprehensive answer to this crisis by the English government certainly exacerbated the issue, but it was not the main cause nor was it genocide. This is often repeated by academics, amongst them is Cormac Ó Gráda - a respected Irish academic. Like /u/Balnibarbian already rightfully pointed out, the potato famine was caused by the Phytophthora infestans. This was a previously unknown type of fungus-like micro-organisms (Oomycota). Scientific tools and knowledge simply fell short to confidently point towards the Phytophtora infestans as the root cause.
This same blight also struck many regions in Central Europe. The failure of crops in some of these regions was comparable to that in Ireland. However, it was very regionally specific and while some regions lost 95% of their crops, others were left mostly unscathed. Belgium, a wealthy and well-connected trade hub, still lost an estimate of 50.000 citizens to famine within the first year or two. This might not seem remotly comparable to the Irish casualties, but you have to take into account the specific circumstances of both regions. The biggest difference is that Belgium recovered within a year or two while Ireland continued to suffer for five to ten years.
So why then didn't Ireland recover as quickly as Belgium did? There are a number of reasons why this happened, but one of them is definitely the isolated position of Ireland. It was not the same versatile and wealthy trade-hub as Belgium, nor did it have the same tradition of strong municipal government combined with a vast transport network. Ireland became was mostly just a "potato-silo" for surrounding regions. The Ireland of 1845 was an Ireland that came into being by playing into the foodmarket, most notably the English one. Its entire economy relied on the export of its produce. So you have an isolated region in a codependant relationship with the English market with its most important crops failing.
It's true that the English government failed to respond quickly and efficiently, but this was not due to malintent. Sure, there were so radical voices shouting racial slurs, but they were mostly a radical minority. You have to remember that this was a time during which "laissez-faire" attitudes reigned supreme. Government intervention was generally frowned upon and this was really obviously a learning process for the English government. Their efforts were too slow, badly informed and badly coordinated. Their incompetence and inexperience showed and was exacerbated by the isolated position of Ireland. In comparison, the Belgium population had a fairly young government with strong municipal roots. They reacted swiftly and with strong measures. Yet they still had to endure a lot of casualties the first year. They also recovered more quickly thanks to local initiatives and help from more fortunate regions.
To conclude, it's true that the English government failed the Irish citizens during the potato famine. However, it wasn't genocide. It was the combination of a natural disaster, incompetence of the English government and the already precarious position of the Irish population. You must realize that this was a time during which the poor had little to fall back upon. Most farmers in Europe were in a similar precarious position. Perhaps not to the same extent as in Ireland, but precarious enough that a disaster of this magnitude would lead to casualties and insurmountable poverty.
I think that Eric VanHaute said it better than I could : "The tensions within the age-old agro-rural society based on ‘commercial peasant economy’ brought the system at the edge during these crisis years. The system cracked, burst, was pushed to and, eventually, over its limits. In the next decades, the system further disintegrated. It became crystal-clear that it lacked the remaining power to sustain a majority of the rural population…".