The adjective unreasonable describes ideas or behaviors that are not guided by reason or sound judgement. It can also refer to things that exceed sensible expectation (a list of unreasonable demands) or that are exorbitant or unconscionable (the prices were unreasonable). Things that are irrational are utterly illogical and may border on the absurd. An irrational fear is one that is not grounded in logic or reason and is therefore unlikely to come true. In mathematics, irrational is used to describe a number that cannot be exactly expressed as a ratio of two integers, such as pi.
An effect is something that is produced by an agency or cause, usually more or less immediately and directly: Exposure to the sun had the effect of toughening his skin. A repercussion is an effect or result, often indirect or remote, of some event or action. Effect carries no ominous baggage or negative associations on its own; it correlates with the word cause as a neutral term describing a relationship between events. But repercussion, which more often appears in the plural, is commonly used to talk about undesirable developments.
The verbs overcome and surmount are close synonyms; they both mean “to prevail over,” as in overcoming or surmounting obstacles or difficulties. Overcome is the more common of the two, and it has meanings that surmount does not. For instance, you can be overcome by grief, that is, overpowered or overwhelmed in body or mind—but you cannot very well be surmounted by grief. But surmount has meanings of its own as well. The earliest senses of the word were “to surpass in excellence” and “to exceed in amount.” While those meanings are obsolete now, the term does retain the suggestion of going above or beyond—in more ways than one. Surmount is also used to talk about getting to the top of things, literally (surmount a hill) and the state of being on top of or above something (a statue surmounting a pillar).