For example...
a thoil!
a chumann!
a ghráidh!
a ghráidh ghil!
a ghealghráidh!
a mhian mo chroí!
a chéadsearc!
a rún!
a shearc!
a rún mo chroí!
a rúnsearc!
a chroí!
a stóirín!
a thaisce!
a chuid!
So much for the romantic part, and then to the hard part. "Hard", yes...The thing which is supposed to become hard, is called...
crann clis or the trick-tree;
bod, pilí, pionna, pruic, rógaire, sáfach, sáiteán, sconnaire, smachtín
or, if it is small or if its owner proves unsatisfactory,
you might call it...
pilibín or cuiteog. The second word actually means "earthworm".
Someone who prefers his own company, and probably that of the físeáin nó irisí pornagrafacha when he's having sex, can at least in Conamara be called a godaí as I have been told by an acquaintance from Rossaveal. What such a man is doing is called lámhchartadh - or, if we want to be quite explicit, tá sé ag tarraingt a bhoid féin.
The female parts involved are called pit or báltaí.
A carnally aroused person is said to have éirí or - if we are talking of a man - adharc, which refers to the cosúlacht of the bod in a hot situation. Both éirí and adharc are said to be upon (ar) you, as emotions and diseases.
A homosexual is called, in an offensive way, piteog and piteachán; both words are connected with pit, obviously enough, and they are also used as invectives for any man found to be effeminate, ineffective etc. The question might be raised whether the usage "effeminate man" is not the primary one, and the "gay" connotation a secondary one. Another word for "homosexual" is peileacán, or "pelican", and the sexual practice known as "sixty-nine" or "soixante-neuf" (in Finnish, incidentally, kuusysi, which is also the name of a predominantly heterosexual football team stationed in the Southern Finnish town of Lahti) is called póg an pheileacáin or "the kiss of the pelican" in Irish. (There is a Finn who is very deeply involved in all kinds of Finnish-Irish activities; when I met him he wanted to have his name translated into Irish, so I facetiously rendered it as Amhlaoibh Ó Peileacáin - I did not yet know about the homosexual connotations of the word peileacán. Now I am horribly afraid of HIS finding out about them.) In fact, the word is probably not thought of as "pelican", but rather as a variant form of feileacán, "butterfly", which is also used to mean a homosexual. Homosexual speakers of Irish should in fact stop using the English calque aerach "gay", and adopt feileacán instead. One native speaker has, by the way, written homo-erotic poetry calling gays na buachaillí bána, i.e. the Whiteboys! I am not sure if this is an old usage in his dialect or his own innovation, but it certainly is a native innovation.
Back to us straightniks, if you don't mind. Sexual intercourse is mostly called bualadh craicinn, déanamh craicinn or streachailt leathair - instead of the Anglicistic usage of the word gnéas (=sex in the sense of the male or the female sex - actually, I suspect this word is a clumsy neologism made by the ligeadóirí agus casadóirí teangan from the Galltacht and not necessarily current in traditional Gaeltacht speech) in the sense "sex[uality]" we should use the good old words craiceann and leathar which are quite unambiguous in such constructions as cúrsaí craicinn, cúrsaí leathair.
More words for the activity: ag déanamh na cuiginne, ag dáthadh, ag slataíocht, ag stialláil, ag tulcadh, ag proiteáil, ag pumpáil, ag pocadh, ag sceilpeáil, ag seomradóireacht, ag marcaíocht.
Men sometimes have long-standing erection without actually being sexually aroused or capable of reaching an orgasm at the moment. This leads to long, unsuccessful, painful, and pleasureless efforts to reach it anyway: such "hammering" is called guicéireacht in Irish.
A fifty-year old man looking for twenty year old girl friends is a pocleandar.